[S390] dasd: prevent validate server for offline devices

Calling validate server on devices in offline processing may cause
an OOPS in the dasd_sleep_on function.

Signed-off-by: Stefan Haberland <stefan.haberland@de.ibm.com>
Reviewed-by: Stefan Weinhuber <wein@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>