s390/ccwgroup: require at least one ccw device

ccwgroup_create_dev() derives the gdev's device name from gdev->cdev[0],
so make sure that this reference is valid.

For robustness only, all current ccwgroup drivers get this right.

Signed-off-by: Julian Wiedmann <jwi@linux.vnet.ibm.com>
Acked-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>