mac80211: add status_driver_data array to ieee80211_tx_info

Drivers might want to have private data in addition
to all other ieee80211_tx_info.status fields.

The current ieee80211_tx_info.rate_driver_data overlaps
with some of the non-rate data (e.g. ampdu_ack_len), so
it might not be good enough.

Since we already know how much free bytes remained,
simply use this size to define (void *) array.

While on it, change ack_signal type from int to the more
explicit s32 type.
Signed-off-by: Eliad Peller <eliadx.peller@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

include/net/mac80211.h

@@ -697,11 +697,11 @@ struct ieee80211_tx_info {
 		} control;
 		struct {
 			struct ieee80211_tx_rate rates[IEEE80211_TX_MAX_RATES];
-			int ack_signal;
+			s32 ack_signal;
 			u8 ampdu_ack_len;
 			u8 ampdu_len;
 			u8 antenna;
-			/* 21 bytes free */
+			void *status_driver_data[21 / sizeof(void *)];
 		} status;
 		struct {
 			struct ieee80211_tx_rate driver_rates[