s390/cio: unregister device when the only path is gone

[ Upstream commit 89c0c62e ]

Currently, if the device is offline and all the channel paths are
either configured or varied offline, the associated subchannel gets
unregistered. Don't unregister the subchannel, instead unregister
offline device.

Signed-off-by: Vineeth Vijayan <vneethv@linux.ibm.com>
Reviewed-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>