ieee80211: mark 802.11 related structs as being 2-byte aligned

Regardless of what header features they use, or if they align the IP
header or not, 802.11 packets from all drivers guarantee a 2-byte
alignment (and there's a debug WARN_ON in case they don't).

Annotate packet structs with __aligned(2) to allow the compiler to use
16-bit load/store operations on platforms with extremely inefficient
unaligned access (e.g. MIPS).

This reduces code size and improves performance on affected platforms
and causes no binary code change on others.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>