s390/css: Fix subchannel detection

s390/css: Fix subchannel detection

We have to consider the m bit to find the real channel subsystem when
determining the last subchannel.

If we fail to take this into account, removal of a subchannel in
the middle of a big list of devices will stop device detection after
a reboot.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Jens Freimann <jfrei@linux.vnet.ibm.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Alexander Graf <agraf@suse.de>