s390x/css: use SubchDev.orb

Instead of passing around a pointer to ORB let us simplify some
function signatures by using the previously introduced ORB saved at the
subchannel (SubchDev).

Signed-off-by: Halil Pasic <pasic@linux.vnet.ibm.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Message-Id: <20170711145441.33925-7-pasic@linux.vnet.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>