mac80211: Stop TX BA session if buf_size is zero

If we receive an ADDBA response with status code 0 and a buf_size of 0
we should stop the TX BA session as otherwise we'll end up queuing
frames in ieee80211_tx_prep_agg forever instead of sending them out as
non AMPDUs.

This fixes a problem with AVM Fritz Stick N wireless devices where
frames to this device are not transmitted anymore by mac80211.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

net/mac80211/agg-tx.c

@@ -777,18 +777,14 @@ void ieee80211_process_addba_resp(struct ieee80211_local *local,
 #ifdef CONFIG_MAC80211_HT_DEBUG
 	printk(KERN_DEBUG "switched off addBA timer for tid %d\n", tid);
 #endif
-
-	if (le16_to_cpu(mgmt->u.action.u.addba_resp.status)
-			== WLAN_STATUS_SUCCESS) {
 	/*
 	 * IEEE 802.11-2007 7.3.1.14:
 	 * In an ADDBA Response frame, when the Status Code field
 	 * is set to 0, the Buffer Size subfield is set to a value
 	 * of at least 1.
 	 */
-		if (!buf_size)
+	if (le16_to_cpu(mgmt->u.action.u.addba_resp.status)
-			goto out;
+			== WLAN_STATUS_SUCCESS && buf_size) {
-
 		if (test_and_set_bit(HT_AGG_STATE_RESPONSE_RECEIVED,
 				     &tid_tx->state)) {
 			/* ignore duplicate response */