iwlagn: fix "Received BA when not expected"

Need to use broadcast sta_id for management frames, otherwise we broke
BA session in the firmware and get messages like that:

"Received BA when not expected"

or (on older kernels):

"BA scd_flow 0 does not match txq_id 10"

This fix regression introduced in 2.6.35 during station management
code rewrite by:

commit 2a87c26bbe9587baeb9e56d3ce0b4971bd777643
Author: Johannes Berg <johannes.berg@intel.com>
Date:   Fri Apr 30 11:30:45 2010 -0700

    iwlwifi: use iwl_find_station less

Patch partially resolve:
https://bugzilla.kernel.org/show_bug.cgi?id=16691
However, there are still 11n performance problems on 4965 and 5xxx
devices that need to be investigated.

Cc: stable@kernel.org # 2.6.35+
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Acked-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/iwlwifi/iwl-agn-tx.c

@@ -568,12 +568,17 @@ int iwlagn_tx_skb(struct iwl_priv *priv, struct sk_buff *skb)
 
 	hdr_len = ieee80211_hdrlen(fc);
 
-	/* Find index into station table for destination station */
-	sta_id = iwl_sta_id_or_broadcast(priv, ctx, info->control.sta);
-	if (sta_id == IWL_INVALID_STATION) {
-		IWL_DEBUG_DROP(priv, "Dropping - INVALID STATION: %pM\n",
-			       hdr->addr1);
-		goto drop_unlock;
+	/* For management frames use broadcast id to do not break aggregation */
+	if (!ieee80211_is_data(fc))
+		sta_id = ctx->bcast_sta_id;
+	else {
+		/* Find index into station table for destination station */
+		sta_id = iwl_sta_id_or_broadcast(priv, ctx, info->control.sta);
+		if (sta_id == IWL_INVALID_STATION) {
+			IWL_DEBUG_DROP(priv, "Dropping - INVALID STATION: %pM\n",
+				       hdr->addr1);
+			goto drop_unlock;
+		}
 	}
 
 	IWL_DEBUG_TX(priv, "station Id %d\n", sta_id);