mac80211: fix A-MSDU outer SA/DA

According to IEEE 802.11-2012 section 8.3.2 table 8-19, the outer SA/DA
of A-MSDU frames need to be changed depending on FromDS/ToDS values.

Signed-off-by: Michael Braun <michael-dev@fami-braun.de>
[use ether_addr_copy and add alignment annotations]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>