vfio/ccw: remove WARN_ON during shutdown

The logic in vfio_ccw_sch_shutdown() always assumed that the input
subchannel would point to a vfio_ccw_private struct, without checking
that one exists. The blamed commit put in a check for this scenario,
to prevent the possibility of a missing private.

The trouble is that check was put alongside a WARN_ON(), presuming
that such a scenario would be a cause for concern. But this can be
triggered by binding a subchannel to vfio-ccw, and rebooting the
system before starting the mdev (via "mdevctl start" or similar)
or after stopping it. In those cases, shutdown doesn't need to
worry because either the private was never allocated, or it was
cleaned up by vfio_ccw_mdev_remove().

Remove the WARN_ON() piece of this check, since there are plausible
scenarios where private would be NULL in this path.

Fixes: 9e6f07cd

 ("vfio/ccw: create a parent struct")
Signed-off-by: Eric Farman <farman@linux.ibm.com>
Reviewed-by: Matthew Rosato <mjrosato@linux.ibm.com>
Link: https://lore.kernel.org/r/20230210174227.2256424-1-farman@linux.ibm.com
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>