[S390] cio: ccw group fix unbind behaviour.

For a ccw group device unbinding it from its driver should do the
same as a call to ungroup, since this virtual device can not exist
without a driver.

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