s390/cio: stop using subchannel_id from ccw_device_private

We want to get rid of the copy of struct subchannel_id maintained in
ccw_device_private, so obtain it from the subchannel directly.

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>