ath9k: Proper padding/unpadding for the TX/RX path.

Software padding is done on the TX path and software unpadding is done on the
RX path. This patch corrects the position where the padding occurs. A specific
function computes the pad position and this function is used in the TX and RX
path. This patch has been tested by generating every possible 802.11 frames
with every possible frame_control field and a varying length. This patch is
useful for analyzing non standard 802.11 frames going over the air

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