vfio: ccw: fix error return in vfio_ccw_sch_event

vfio: ccw: fix error return in vfio_ccw_sch_event

[ Upstream commit 2c861d89ccda2fbcea9358eff9cc5f8fae548be5 ]

If the device has not been registered, or there is work pending,
we should reschedule a sch_event call again.

Signed-off-by: Dong Jia Shi <bjsdjshi@linux.vnet.ibm.com>
Message-Id: <20180502072559.50691-1-bjsdjshi@linux.vnet.ibm.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>