[S390] cio: Introduce ccw_bus_type.shutdown.

Introduce a shutdown method for the ccw bus that calls the driver
specific shutdown method in struct ccw_driver.
Switch zfcp to the new ccw_driver shutdown method.

Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>