ath9k: Run the LNA combining algorithm properly

The LNA combining algorithm has to be run for cards
that support the required diversity features, make
sure that that correct conditions are met before
enabing this algorithm.

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