ath9k: Fix descriptor length for AR9462

Change the descriptor length to 24 and explicitly
set the control field 23 to zero. Not doing so would
result in dropping of frames.

Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>