ath9k_hw: improve performance for AR934x v1.3+

AR934x v1.3 no longer needs the DCU backoff reduction workaround for
preventing rx overruns, but in turn needs the number of usable Tx
buffers to be reduced slightly.

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/hw.c b/drivers/net/wireless/ath/ath9k/hw.c
index 01e97ce..15dfefc 100644
--- a/drivers/net/wireless/ath/ath9k/hw.c
+++ b/drivers/net/wireless/ath/ath9k/hw.c
@@ -1172,6 +1172,7 @@
 static inline void ath9k_hw_set_dma(struct ath_hw *ah)
 {
 	struct ath_common *common = ath9k_hw_common(ah);
+	int txbuf_size;
 
 	ENABLE_REGWRITE_BUFFER(ah);
 
@@ -1225,13 +1226,17 @@
 		 * So set the usable tx buf size also to half to
 		 * avoid data/delimiter underruns
 		 */
-		REG_WRITE(ah, AR_PCU_TXBUF_CTRL,
-			  AR_9285_PCU_TXBUF_CTRL_USABLE_SIZE);
-	} else if (!AR_SREV_9271(ah)) {
-		REG_WRITE(ah, AR_PCU_TXBUF_CTRL,
-			  AR_PCU_TXBUF_CTRL_USABLE_SIZE);
+		txbuf_size = AR_9285_PCU_TXBUF_CTRL_USABLE_SIZE;
+	} else if (AR_SREV_9340_13_OR_LATER(ah)) {
+		/* Uses fewer entries for AR934x v1.3+ to prevent rx overruns */
+		txbuf_size = AR_9340_PCU_TXBUF_CTRL_USABLE_SIZE;
+	} else {
+		txbuf_size = AR_PCU_TXBUF_CTRL_USABLE_SIZE;
 	}
 
+	if (!AR_SREV_9271(ah))
+		REG_WRITE(ah, AR_PCU_TXBUF_CTRL, txbuf_size);
+
 	REGWRITE_BUFFER_FLUSH(ah);
 
 	if (AR_SREV_9300_20_OR_LATER(ah))