mac80211: fix alignment calculation bug

When checking whether or not a given frame needs to be
moved to be properly aligned to a 4-byte boundary, we
use & 4 which wasn't intended, this code should check
the lowest two bits.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>