ath9k: Fix interface accounting

Currently, the interface count is maintained globally,
but this causes problems in RX filter calculation.
Make the interface count a per-channel-context variable
to fix this.

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