s390/cio: remove subchannel_id from ccw_device_private

A copy of struct subchannel_id is maintained in ccw_device_private.
The subchannel id is a property of the subchannel. The additional
copy is not needed.

Internal users can obtain it from subchannel.schid - device drivers
can use ccw_device_get_schid().

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