ath9k: This patch fix RX unpadding for any received frame.

It has been tested with a 802.11 frame generator and by checking the FCS field
of each received frame with the value reported by the Atheros hardware. This
patch is useful if you are trying to analyze non standard 802.11 frame going
over the air.

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