ath9k: Fix RX filters in channel contexts

Maintain the RX filter on a per-channel-context
basis and not globally. Not doing so was resulting
in incorrect filter calculation.

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