ath9k: Fix baseband watchdog interrupts

Program the required baseband watchdog interrupt
mask to ensure that the correct watchdog interrupts
are raised when the BB is hung for some reason.

Also, use the capability HW_BB_WATCHDOG instead of
relying on other flags.

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