s390/zcrypt: fix device unregister race

The ap_queue_messsage function will call device_unregister if the
unregistered field of the device has been set while trying to queue
a message. This races with other device_unregister calls, e.g. from
the ap_scan_bus. Remove the call to device_unregister from
ap_queue_message and let ap_scan_bus deal with it.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>