ath9k: downgrade ASSERT() in ath_clone_txbuf()

We can easily run out of tx buf if there is any stuck in
transmission, so downgrade it to WARN_ON().
Signed-off-by: NVasanthakumar Thiagarajan <vasanth@atheros.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>

drivers/net/wireless/ath/ath9k/xmit.c

@@ -238,6 +238,10 @@ static struct ath_buf* ath_clone_txbuf(struct ath_softc *sc, struct ath_buf *bf)
 	struct ath_buf *tbf;
 
 	spin_lock_bh(&sc->tx.txbuflock);
+	if (WARN_ON(list_empty(&sc->tx.txbuf))) {
+		spin_unlock_bh(&sc->tx.txbuflock);
+		return NULL;
+	}
 	ASSERT(!list_empty((&sc->tx.txbuf)));
 	tbf = list_first_entry(&sc->tx.txbuf, struct ath_buf, list);
 	list_del(&tbf->list);
@@ -379,6 +383,8 @@ static void ath_tx_complete_aggr(struct ath_softc *sc, struct ath_txq *txq,
 				struct ath_buf *tbf;
 
 				tbf = ath_clone_txbuf(sc, bf_last);
+				if (!tbf)
+					break;
 				ath9k_hw_cleartxdesc(sc->sc_ah, tbf->bf_desc);
 				list_add_tail(&tbf->list, &bf_head);
 			} else {