ath9k: fix crashes when using shared IRQs

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

Backport of r43239

git-svn-id: svn://svn.openwrt.org/openwrt/branches/barrier_breaker@43240 3c298f89-4303-0410-b956-a3cf2f4a3e73