ath9k: Handle FATAL interrupts correctly

When a FATAL interrupt is received, a full chip reset is
required, which is done in the main tasklet. But since
the reset routine is scheduled as a work item, make sure
that interrupts are not enabled in the tasklet before the
reset is done.

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