ath9k: Add a HW capability for WLAN/BT RX diversity

Make use of this capability to restrict the usage of the
debugfs file and modparam using which this feature can
be enabled.

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