ath9k: Dont update rate control for every AMPDU

Update the rate control only with the tx status of first
AMPDU of an aggregation. This patch fixes frequent drops
in throughput.

Signed-off-by: Vasanthakumar Thiagarajan <vasanth@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
diff --git a/drivers/net/wireless/ath9k/rc.c b/drivers/net/wireless/ath9k/rc.c
index e118824..7d1913d 100644
--- a/drivers/net/wireless/ath9k/rc.c
+++ b/drivers/net/wireless/ath9k/rc.c
@@ -1413,7 +1413,8 @@
 	an = (struct ath_node *)sta->drv_priv;
 	final_ts_idx = tx_info_priv->tx.ts_rateindex;
 
-	if (!an || !priv_sta || !ieee80211_is_data(fc))
+	if (!an || !priv_sta || !ieee80211_is_data(fc) ||
+	    !tx_info_priv->update_rc)
 		goto exit;
 
 	if (tx_info_priv->tx.ts_status & ATH9K_TXERR_FILT)