[S390] tape: Use ccw_dev_id to build cdev_id.

To construct the integer containing the information from the bus_id,
it is easier to use the data from ccw_dev_id than to parse the bus_id.

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