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/txrx.c

@@ -1957,18 +1957,18 @@ void bes2600_rx_cb(struct bes2600_vif *priv,
 		 * path is taken.
 		 */
 		if (hw_priv->bes_power.pm_unsupported) {
-			ieee80211_rx_irqsafe(priv->hw, skb);
+			ieee80211_rx_ni(priv->hw, skb);
 		} else {
 			spin_lock_bh(&priv->ps_state_lock);
 			/* Double-check status with lock held */
 			if (entry->status == BES2600_LINK_SOFT)
 				skb_queue_tail(&entry->rx_queue, skb);
 			else
-				ieee80211_rx_irqsafe(priv->hw, skb);
+				ieee80211_rx_ni(priv->hw, skb);
 			spin_unlock_bh(&priv->ps_state_lock);
 		}
 	} else {
-		ieee80211_rx_irqsafe(priv->hw, skb);
+		ieee80211_rx_ni(priv->hw, skb);
 	}
 	*skb_p = NULL;