s390/cio: Do not unregister the subchannel based on DNV

[ Upstream commit 8c58a229 ]

Starting with commit 2297791c ("s390/cio: dont unregister
subchannel from child-drivers"), CIO does not unregister subchannels
when the attached device is invalid or unavailable. Instead, it
allows subchannels to exist without a connected device. However, if
the DNV value is 0, such as, when all the CHPIDs of a subchannel are
configured in standby state, the subchannel is unregistered, which
contradicts the current subchannel specification.

Update the logic so that subchannels are not unregistered based
on the DNV value. Also update the SCHIB information even if the
DNV bit is zero.

Suggested-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Signed-off-by: Vineeth Vijayan <vneethv@linux.ibm.com>
Fixes: 2297791c ("s390/cio: dont unregister subchannel from child-drivers")
Reviewed-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>