bes2600: filter 5 GHz scans at the driver boundary (besser#1)

The BES2600 firmware refuses WSM start-scan for 5 GHz with status 2
("rejected by policy").  This shows up in dmesg as the recurring

    wsm_generic_confirm failed for request 0x0007.
    [SCAN] Scan failed (-22).

pattern (besser issue #1, ~14-16/h on ohm/PineTab2 baseline).

Trace shows every reject is the second of a back-to-back pair: mac80211
splits multi-band hw_scan requests per band when the driver does not
set IEEE80211_HW_SINGLE_SCAN_ON_ALL_BANDS (we don't), then re-invokes
drv_hw_scan from __ieee80211_scan_completed for each subsequent band.
The 2.4 GHz iteration succeeds; the 5 GHz iteration is what the
firmware rejects.  See ieee80211_prep_hw_scan in net/mac80211/scan.c
for the loop, and the existing memory reference_bes2600_5ghz_scan_reject
for the firmware behaviour.

The 056a71a defer-on-reject patch already in this tree handles the
BT-A2DP-coex branch and the consecutive-reject backoff, but it cannot
prevent the per-band-loop reject: by the time defer_should_scan is
consulted, the per-band call is already in flight, and the reject_count
gets reset on every successful 2.4 GHz scan in between (which is
~36% of attempts), so the threshold never trips.

The fix: refuse the 5 GHz iteration upfront in bes2600_hw_scan.  The
2.4 GHz scan still runs normally.  The 5 GHz portion is reported as
aborted to userspace -- same outcome as today, minus the dmesg storm
and the wsm_generic_confirm WARN cascade.

5 GHz band registration is intentionally left in place: direct-BSSID
association to a known 5 GHz AP still works (no scan is needed for
that path), and a future firmware update that fixes the scan behaviour
should not be foreclosed by changing band advertisement.

Contract: per include/net/mac80211.h ieee80211_ops.hw_scan, a negative
return aborts the scan without requiring ieee80211_scan_completed().
-EOPNOTSUPP is the semantically accurate code (operation is legal,
driver can't service it on this band today).

Phase 3 evidence:
- baseline N=3: rate ~14.3-23.6/h converged at 14.3/h (matches OP)
- back-to-back scan gap: 6/6 rejected pairs <200us, 1/1 successful
  pair was 114ms (single-band-only, no 5 GHz leg)
- defer log fires: 0/9 in 30-min window (056a71a structurally bypassed)

Predicted Phase 7 delta: Pattern A 14/h -> 0/h.

bes2600/scan.c

@@ -238,6 +238,28 @@ int bes2600_hw_scan(struct ieee80211_hw *hw,
 	/* Scan when P2P_GO corrupt firmware MiniAP mode */
 	if (priv->join_status == BES2600_JOIN_STATUS_AP)
 		return -EOPNOTSUPP;
+
+	/*
+	 * Firmware refuses WSM start-scan for 5 GHz with status 2 ("rejected
+	 * by policy"); see besser issue #1.  mac80211 splits multi-band
+	 * hw_scan requests per-band when the driver does not set
+	 * IEEE80211_HW_SINGLE_SCAN_ON_ALL_BANDS (we don't -- see
+	 * ieee80211_hw_set() calls in bes2600_main.c), so each per-band call
+	 * has req->channels[] from one band only (see ieee80211_prep_hw_scan
+	 * in net/mac80211/scan.c).  Refuse the 5 GHz iteration at the driver
+	 * boundary so userspace gets a clean aborted-scan for that portion
+	 * rather than waiting for the firmware reject to cascade up.  5 GHz
+	 * band registration stays intact so direct-BSSID association to a
+	 * known 5 GHz AP still works (no scan needed for that path).
+	 *
+	 * Contract: per include/net/mac80211.h struct ieee80211_ops.hw_scan
+	 * documentation, a negative return aborts the scan without requiring
+	 * ieee80211_scan_completed().
+	 */
+	if (req->n_channels > 0 &&
+	    req->channels[0]->band == NL80211_BAND_5GHZ)
+		return -EOPNOTSUPP;
+
 #if 0
 	if (work_pending(&priv->offchannel_work) ||
 			(hw_priv->roc_if_id != -1)) {