ath9k: Fix interface combinations for multi-channel concurrency

Currently mac80211 does not support WDS and DFS with channel context
drivers. So advertise these features only when the driver is not
supporting channel context and modparam "use_chanctx" is introduced
for preparing channel context support in ath9k.

Signed-off-by: Rajkumar Manoharan <rmanohar@qti.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>