ath9k_hw: Fix rx latency of 11a mode

Rx latecy to start signal(usec) of 11a is 41 not 37 and
also corrected the rx delay in quarter rate.

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