[S390] cio: Reset sch->driver.

sch->driver needs to be reset to NULL on failed probe and after
remove. We also need to check for sch->driver on shutdown.

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