mac80211: add explicit padding in struct ieee80211_tx_info

Otherwise, the BUILD_BUG_ON calls in ieee80211_tx_info_clear_status can
fail on some architectures.
Signed-off-by: John W. Linville <linville@tuxdriver.com>

include/net/mac80211.h

@@ -341,6 +341,7 @@ struct ieee80211_tx_info {
 	u8 antenna_sel_tx;
 
 	/* 2 byte hole */
+	u8 pad[2];
 
 	union {
 		struct {