virtio_ccw: fix vcdev pointer handling issues

The interrupt handler virtio_ccw_int_handler() using the vcdev pointer
is protected by the ccw_device lock. Resetting the pointer within the
ccw_device structure should be done when holding this lock.

Also resetting the vcdev pointer (under the ccw_device lock) prior to
freeing the vcdev pointer memory removes a critical path.

Signed-off-by: Heinz Graalfs <graalfs@linux.vnet.ibm.com>
Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>