iwlwifi: stop interrupts before stopping device

If an RX interrupt is signalled after or during apm_stop
we may try to access the peripherals which are already
down. Prevent this by disabling interrupts first.

Signed-off-by: David Spinadel <david.spinadel@intel.com>
Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>