ath9k: Handle fatal interrupts properly

When a fatal interrupt is received or it is detected that the baseband
has hung, the chip has to be reset immediately.  Otherwise, we end up
processing spurious interrupts. Ensure that we bail out properly in
the ISR when the reset work hasn't completed yet.

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