mac80211: fix purging multicast PS buffer queue

The code currently assumes that buffered multicast PS frames don't have
a pending ACK frame for tx status reporting.
However, hostapd sends a broadcast deauth frame on teardown for which tx
status is requested. This can lead to the "Have pending ack frames"
warning on module reload.
Fix this by using ieee80211_free_txskb/ieee80211_purge_tx_queue.

Cc: stable@vger.kernel.org
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
diff --git a/net/mac80211/cfg.c b/net/mac80211/cfg.c
index 47e99ab8..543b1d4 100644
--- a/net/mac80211/cfg.c
+++ b/net/mac80211/cfg.c
@@ -869,7 +869,7 @@
 
 	/* free all potentially still buffered bcast frames */
 	local->total_ps_buffered -= skb_queue_len(&sdata->u.ap.ps.bc_buf);
-	skb_queue_purge(&sdata->u.ap.ps.bc_buf);
+	ieee80211_purge_tx_queue(&local->hw, &sdata->u.ap.ps.bc_buf);
 
 	mutex_lock(&local->mtx);
 	ieee80211_vif_copy_chanctx_to_vlans(sdata, true);