s390x/css: use ccw data stream

Replace direct access which implicitly assumes no IDA
or MIDA with the new ccw data stream interface which should
cope with these transparently in the future.

Note that checking the return code for ccw_dstream_* will be
done in a follow-on patch.

Signed-off-by: Halil Pasic <pasic@linux.vnet.ibm.com>
Reviewed-by: Dong Jia Shi <bjsdjshi@linux.vnet.ibm.com>
Reviewed-by: Pierre Morel <pmorel@linux.vnet.ibm.com>
Message-Id: <20170921180841.24490-3-pasic@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>
