mac80211: Add tx ack signal support in sta info

This allows users to get ack signal strength of
last transmitted frame.

Signed-off-by: Venkateswara Naralasetty <vnaralas@codeaurora.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
diff --git a/include/net/mac80211.h b/include/net/mac80211.h
index 906e902..854037b 100644
--- a/include/net/mac80211.h
+++ b/include/net/mac80211.h
@@ -934,6 +934,7 @@ struct ieee80211_tx_info {
 			u8 ampdu_len;
 			u8 antenna;
 			u16 tx_time;
+			bool is_valid_ack_signal;
 			void *status_driver_data[19 / sizeof(void *)];
 		} status;
 		struct {