mac80211: fix BUILD_BUG_ON() caused by misalignment on arm

On ARM alignment is done slightly different from other architectures.
struct ieee80211_tx_rate is aligned to word size, even though it only has 3
single-byte members, which triggers the BUILD_BUG_ON in
ieee80211_tx_info_clear_status

This patch marks the struct ieee80211_tx_rate as packed, so that ARM
behaves like the other architectures.
Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

include/net/mac80211.h

@@ -307,7 +307,7 @@ struct ieee80211_tx_rate {
 	s8 idx;
 	u8 count;
 	u8 flags;
-};
+} __attribute__((packed));
 
 /**
  * struct ieee80211_tx_info - skb transmit information