ath9k_hw: Read and configure xpa timing field

Configure xpa timing field while loading boad defaults to
fix 11b CCK spur issue that was observed in EMI testing.

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