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>

drivers/net/wireless/ath/carl9170/tx.c

@@ -277,11 +277,11 @@ static void carl9170_tx_release(struct kref *ref)
 		return;
 
 	BUILD_BUG_ON(
-	    offsetof(struct ieee80211_tx_info, status.ampdu_ack_len) != 23);
+	    offsetof(struct ieee80211_tx_info, status.ack_signal) != 20);
 
-	memset(&txinfo->status.ampdu_ack_len, 0,
+	memset(&txinfo->status.ack_signal, 0,
 	       sizeof(struct ieee80211_tx_info) -
-	       offsetof(struct ieee80211_tx_info, status.ampdu_ack_len));
+	       offsetof(struct ieee80211_tx_info, status.ack_signal));
 
 	if (atomic_read(&ar->tx_total_queued))
 		ar->tx_schedule = true;