vfio/ccw: Check return code from subchannel quiesce

If a subchannel is busy when a close is performed, the subchannel
needs to be quiesced and left nice and tidy, so nothing unexpected
(like a solicited interrupt) shows up while in the closed state.
Unfortunately, the return code from this call isn't checked,
so any busy subchannel is treated as a failing one.

Fix that, so that the close on a busy subchannel happens normally.

Signed-off-by: Eric Farman <farman@linux.ibm.com>
Reviewed-by: Matthew Rosato <mjrosato@linux.ibm.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/20220728204914.2420989-4-farman@linux.ibm.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>