mac80211: use 16 bit alignment for the if_ibss bssid field

Several MAC address comparison functions assume 16 bit alignment for pointers
passed to them. Since the addition of the control_port field, alignment
for the IBSS bssid was off by one, causing a severe performance hit on
architectures without efficient unaligned access (e.g. MIPS).

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>