ath9k: do not access hardware on IRQs during reset

Instead of killing interrupts during reset when the first one happens,
kill them before issuing the reset.
This fixes an easy to reproduce crash with multiple cards sharing the
same IRQ.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>