[S390] ccwgroup: cleanup

Fix coding style, remove forward declerations, simplify code.
Also remove a superfluous get_device/put_device pair in
ccwgroup_create_from_string.

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