ieee80211: fix unaligned access in ieee80211_copy_snap

There is no guarantee that data+SNAP_SIZE will reside on an even numbered
address, so doing a 16 bit read will cause an unaligned access in some
situations. Based on a patch from Jun Sun.

Signed-off-by: Daniel Drake <dsd@gentoo.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>