[S390] cio: Fix I/O subchannel refcounting.

[S390] cio: Fix I/O subchannel refcounting.

Subchannel refcounting was incorrect in some places, especially
a refcount was missing when ccw_device_call_sch_unregister()
was called and the refcount was not correctly switched after
moving devices.

Fix this by establishing the following rules:
- The ccw_device obtains a reference on its parent subchannel
  when dev.parent is set and gives it up in its release
  function. This is needed because we need a parent reference
  for correct refcounting even before the ccw device is (if at
  all) registered.
- When calling device_move(), obtain a reference on the new
  subchannel before moving the ccw device and give up the
  reference on the old parent after moving. This brings the
  refcount in line with the first rule.

Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>