ath9k: change the default antenna settings based on diversity

change the AR_DEF_ANTENNA register settings i.e setting default antenna
setting only for antenna diversity enabled chipsets. no point in
doing this for MIMO chipsets

Signed-off-by: Mohammed Shafi Shajakhan <mohammed@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>