From: Martin Schwidefsky <schwidefsky@de.ibm.com>

Common i/o layer changes:
 - Avoid de-registering a ccwgroup device multiple times.
 - Remove check for channel path objects in get_subchannel_by_schid.
   Channel patch objects are never in the bus list.
 - Avoid NULL pointer deref. in qdio_unmark_q.
 - Fix reference counting on subchannel objects.
 - Add shutdown function to terminate i/o and disable subchannels at reipl.
 - Remove all ccwgroup devices if the ccwgroup driver is unregistered.