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 <[e-mail]>
Acked-by: Johannes Berg <[e-mail]>
Signed-off-by: John W. Linville <[e-mail]>

net/mac80211/agg-tx.c

···
 #ifdef CONFIG_MAC80211_HT_DEBUG
 	printk(KERN_DEBUG "switched off addBA timer for tid %d\n", tid);
 #endif
-
+	/*
+	 * 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 (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)
-			goto out;
-
+			== WLAN_STATUS_SUCCESS && buf_size) {
 		if (test_and_set_bit(HT_AGG_STATE_RESPONSE_RECEIVED,
 				     &tid_tx->state)) {
 			/* ignore duplicate response */