ath9k: fix a WARN_ON when aggregation start is issued more than once, should improve stability with 802.11n

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@22827 3c298f89-4303-0410-b956-a3cf2f4a3e73

package/mac80211/patches/580-ath9k_aggr_start_fix.patch

@@ -0,0 +1,12 @@
+--- a/drivers/net/wireless/ath/ath9k/xmit.c
++++ b/drivers/net/wireless/ath/ath9k/xmit.c
+@@ -124,7 +124,8 @@ static void ath_tx_resume_tid(struct ath
+ {
+ 	struct ath_txq *txq = &sc->tx.txq[tid->ac->qnum];
+ 
+-	WARN_ON(!tid->paused);
++	if (!tid->paused)
++		return;
+ 
+ 	spin_lock_bh(&txq->axq_lock);
+ 	tid->paused = false;