ath9k: Set SWCOM value for CUS198

CUS198/CUS230 cards require a custom value to be
programmed into the SWCOM register. Assign this during
init time.

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