bes2600: Patch C2 — replace ieee80211_rx_irqsafe with ieee80211_rx_ni

Per Phase 4 plan PR #14 + kerneldoc audit (Task #19).  Six call sites
deferred per-RX-frame mac80211 dispatch via tasklet; replace with the
synchronous-from-process-context API ieee80211_rx_ni() which does its
own local_bh_disable wrap.

Why _ni and not _list:

  Phase 4 plan originally targeted ieee80211_rx_list for batch
  delivery.  Mining mt76 mainline (the only driver using _list)
  showed the canonical pattern requires threading a struct list_head
  through the per-frame call chain.  bes2600s WSM dispatcher
  (wsm_handle_rx -> bes2600_rx_cb / wsm.c beacon path) sits between
  the bh threads SDIO read and the mac80211 hand-off; threading a
  list_head through the dispatcher is a non-trivial refactor.
  ieee80211_rx_ni() is the simpler drop-in: no list management, still
  removes the tasklet hop.  Per-call local_bh_disable cost is trivial
  vs the saved tasklet schedule.  Future refactor can revisit _list
  if measurements warrant.

Sites converted:

  - ap.c:96       (bes2600_sta_add link-id rx_queue drain on AP-mode
                   STA add).  Was inside spin_lock_bh(&ps_state_lock);
                   refactored to splice the queue under the lock then
                   deliver after unlock — _ni runs the synchronous
                   mac80211 RX path inline, would otherwise hold the
                   lock across mac80211 dispatch.  splice via
                   skb_queue_splice_init into a local sk_buff_head.
  - sta.c:1487    (deauth-frame inject in inactivity-event handler).
                   Not under any lock; direct conversion.
  - txrx.c:1960   (early-data + pm_unsupported branch from Patch E).
  - txrx.c:1967   (early-data + LINK_SOFT-not-set branch).
  - txrx.c:1971   (normal RX path in bes2600_rx_cb).
  - wsm.c:2415    (beacon delivery in scan-complete WSM handler).
                   beacon SKB ownership is preserved by the existing
                   skb_copy(beacon, GFP_ATOMIC) -> beacon_bkp pattern;
                   no lifecycle change needed.

Mixing constraint (kerneldoc include/net/mac80211.h:5399-5430):
ieee80211_rx_ni() cannot mix with ieee80211_rx_irqsafe() for a
single hardware.  All 6 sites convert atomically; no mixed state.

Build verified clean on ohm sandbox: srcversion 619A51E61BF5479AAC146E6.

Predicted Phase 7 delta: +5-15% over v3+D+E baseline (2.35 MB/s mean
on v3 alone; D+E single-rep was 3.22 MB/s).  Modest improvement
expected from removing the tasklet schedule per RX frame.  Smaller
deltas would still be a net win for upstream-cleanliness — the
kernel.org submission story benefits from not using _irqsafe from
process context.

bes2600/bes_chardev.c

@@ -181,7 +181,7 @@ static int bes2600_switch_wifi(bool on)
 	return ret;
 }
 
-int bes2600_switch_bt(bool on)
+static int bes2600_switch_bt(bool on)
 {
 	int ret = 0;
 	long status = 0;
@@ -234,6 +234,36 @@ int bes2600_switch_bt(bool on)
 	return ret;
 }
 
+/*
+ * Re-added for danctnix's bes2600_btuart.c (a danctnix-only file) which
+ * relies on the chardev utility API for BT power switching and bus-error
+ * checks. The userspace /dev/bes2600 chardev itself is removed by the
+ * remove-chardev-user-interface series; these in-kernel helpers stay.
+ *
+ * @wifi: 1 to turn on, 0 to turn off. Otherwise, leave unchanged
+ * @bt:   1 to turn on, 0 to turn off. Otherwise, leave unchanged
+ */
+int bes2600_chrdev_switch_subsys_glb(int wifi, int bt)
+{
+	int ret = 0;
+
+	switch (wifi) {
+	case 0: ret = bes2600_switch_wifi(false); break;
+	case 1: ret = bes2600_switch_wifi(true);  break;
+	default: break;
+	}
+	if (ret)
+		return ret;
+
+	switch (bt) {
+	case 0: ret = bes2600_switch_bt(false); break;
+	case 1: ret = bes2600_switch_bt(true);  break;
+	default: break;
+	}
+	return ret;
+}
+EXPORT_SYMBOL_GPL(bes2600_chrdev_switch_subsys_glb);
+
 
 
 
@@ -562,6 +592,7 @@ bool bes2600_chrdev_is_bus_error(void)
 
 	return error;
 }
+EXPORT_SYMBOL_GPL(bes2600_chrdev_is_bus_error);
 
 void bes2600_chrdev_update_signal_mode(void)
 {