mac80211: round IEEE80211_TX_STATUS_HEADROOM up to multiple of 4

This ensures that mac80211 allocated management frames are properly
aligned, which makes copying them more efficient.
For instance, mt76 uses iowrite32_copy to copy beacon frames to beacon
template memory on the chip.
Misaligned 32-bit accesses cause CPU exceptions on MIPS and should be
avoided.
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

include/net/mac80211.h

@@ -4141,7 +4141,7 @@ void ieee80211_sta_uapsd_trigger(struct ieee80211_sta *sta, u8 tid);
  * The TX headroom reserved by mac80211 for its own tx_status functions.
  * This is enough for the radiotap header.
  */
-#define IEEE80211_TX_STATUS_HEADROOM	14
+#define IEEE80211_TX_STATUS_HEADROOM	ALIGN(14, 4)
 
 /**
  * ieee80211_sta_set_buffered - inform mac80211 about driver-buffered frames