ath9k: fix interrupt storms on queued hardware reset

commit b74713d0
"ath9k: Handle fatal interrupts properly" introduced a race condition, where
IRQs are being left enabled, however the irq handler returns IRQ_HANDLED
while the reset is still queued without addressing the IRQ cause.
This leads to an IRQ storm that prevents the system from even getting to
the reset code.

Fix this by disabling IRQs in the handler without touching intr_ref_cnt.

Cc: Rajkumar Manoharan <rmanohar@qca.qualcomm.com>
Cc: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>