s390/qdio: obtain subchannel_id via ccw_device_get_schid()

We want to get rid of the copy of struct subchannel_id maintained in
ccw_device_private, so obtain it using ccw_device_get_schid().

Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Reviewed-by: Ursula Braun <ubraun@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>