[S390] cio: Use device_is_registered().

Check if a ccw device is registered via device_is_registered()
and not via the old kludge of checking the membership in driver
core internal klists.

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