ath9k: prevent early IRQs from accessing hardware

IRQs are suppressed if ah == NULL and ATH_OP_INVALID being set in
common->op_flags. Close a short time window between those two.

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