mac80211: reduce IEEE80211_TX_MAX_RATES

IEEE80211_TX_MAX_RATES can be reduced from 5 to 4 as there
is no current hardware supporting a rate chain with 5 multi
rate stages (mrr), so 4 mrr stages are sufficient.

The memory that is freed within the ieee80211_tx_info struct
will be used in the upcoming Transmission Power Control (TPC)
implementation.

Suggested-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Thomas Huehn <thomas@net.t-labs.tu-berlin.de>
[reword commit message]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
diff --git a/include/net/mac80211.h b/include/net/mac80211.h
index dc2a97a..3f1b58c 100644
--- a/include/net/mac80211.h
+++ b/include/net/mac80211.h
@@ -475,7 +475,7 @@
 #define IEEE80211_TX_INFO_RATE_DRIVER_DATA_SIZE 24
 
 /* maximum number of rate stages */
-#define IEEE80211_TX_MAX_RATES	5
+#define IEEE80211_TX_MAX_RATES	4
 
 /**
  * struct ieee80211_tx_rate - rate selection/status
@@ -563,11 +563,11 @@
 		} control;
 		struct {
 			struct ieee80211_tx_rate rates[IEEE80211_TX_MAX_RATES];
-			u8 ampdu_ack_len;
 			int ack_signal;
+			u8 ampdu_ack_len;
 			u8 ampdu_len;
 			u8 antenna;
-			/* 14 bytes free */
+			/* 21 bytes free */
 		} status;
 		struct {
 			struct ieee80211_tx_rate driver_rates[
@@ -634,7 +634,7 @@
 		info->status.rates[i].count = 0;
 
 	BUILD_BUG_ON(
-	    offsetof(struct ieee80211_tx_info, status.ampdu_ack_len) != 23);
+	    offsetof(struct ieee80211_tx_info, status.ack_signal) != 20);
 	memset(&info->status.ampdu_ack_len, 0,
 	       sizeof(struct ieee80211_tx_info) -
 	       offsetof(struct ieee80211_tx_info, status.ampdu_ack_len));