ath9k: Remove MAC header pad before reporting TX status

Remove the possible MAC header pad before reporting TX status to
mac80211. This pad is hardware specific operation and should not be
exposed outside the driver. This fixes the frame body in monitor
interfaces that could be used to check on TX status for transmitted
frames.

Signed-off-by: Jouni Malinen <jouni.malinen@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>