ath9k: optimize ath_drain_all_txq

If the software has processed all packets, checking the hardware queue
is unnecessary.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
diff --git a/drivers/net/wireless/ath/ath9k/xmit.c b/drivers/net/wireless/ath/ath9k/xmit.c
index 95f6c5a..24846d9 100644
--- a/drivers/net/wireless/ath/ath9k/xmit.c
+++ b/drivers/net/wireless/ath/ath9k/xmit.c
@@ -1786,6 +1786,9 @@
 		if (!ATH_TXQ_SETUP(sc, i))
 			continue;
 
+		if (!sc->tx.txq[i].axq_depth)
+			continue;
+
 		if (ath9k_hw_numtxpending(ah, sc->tx.txq[i].axq_qnum))
 			npend |= BIT(i);
 	}