ath9k: Support ANT diversity for WB225

WB225 based cards like CUS198 and CUS230 support
both fast antenna diversity and LNA combining. Add support
for this and also program the SWCOM register with the
correct "ant_ctrl_comm2g_switch_enable" value.

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