s390/ccwgroup: add get_ccwgroupdev_by_busid()

Provide function to find a ccwgroup device by its busid.

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