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>