mac80211: fix alignment warning

When I introduced the alignment warning I forgot the A-MSDU case which
has a different requirement because each frame contains 14-byte 802.3
headers in front of the IP payload. This patch moves the alignment
warning to a place where we know whether we're dealing with an A-MSDU
frame and adjusts it accordingly.

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