s390/cio: include subchannels without devices also for evaluation

Currently when the new channel-path is enabled, we do evaluation only
on the subchannels with a device connected on it. This is because,
in the past, if the device in the subchannel is not working or not
available, we used to unregister the subchannels. But, from the 'commit
2297791c ("s390/cio: dont unregister subchannel from child-drivers")'
we allow subchannels with or without an active device connected
on it. So, when we do the io_subchannel_verify, make sure that,
we are evaluating the subchannels without any device too.

Fixes: 2297791c

 ("s390/cio: dont unregister subchannel from child-drivers")
Reported-by: Boris Fiuczynski <fiuczy@linux.ibm.com>
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>