iwlwifi: quiet a noisy printk

Timing issues in microcode for some devices can cause a compressed BA to
be sent to the driver prior to returning any a-MPDU notification.
Traces show RTS-CTS is exchanged and then the timer fires which causes an
empty BA to be sent which acknowledges nothing.  This results in a noisy
printk. Only print the message if the bitmap is non-zero.

Signed-off-by:  Don Fry <donald.h.fry@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/iwlwifi/iwl-agn-tx.c

@@ -1227,7 +1227,8 @@ static int iwlagn_tx_status_reply_compressed_ba(struct iwl_priv *priv,
 	struct ieee80211_tx_info *info;
 
 	if (unlikely(!agg->wait_for_ba))  {
-		IWL_ERR(priv, "Received BA when not expected\n");
+		if (unlikely(ba_resp->bitmap))
+			IWL_ERR(priv, "Received BA when not expected\n");
 		return -EINVAL;
 	}