ath5k: fix 802.11 header padding on RX, unpadding on TX

Padding the 802.11 header to a multiple of 4 bytes needs to be done only for
frames with a body. This fixes a bug where 2 bytes were missing in monitor
mode for ACK frames. Inspired by a patch from Jouni Malinen on ath9k.

Ref: http://bugzilla.kernel.org/show_bug.cgi?id=12101

 :
Signed-off-by: Benoit Papillault <benoit.papillault@free.fr>
Signed-off-by: John W. Linville <linville@tuxdriver.com>