ath9k_htc: Advertise interface combinations supported

This will allow us to create virtual interface the driver supports.
Also this ensures multivif support and limitation advertised
by the driver is taken care in cfg80211 itself.

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