s390x/css: reflect cpa in scsw

We neglected to update the the channel-program-address field of the scsw
after completion of the start or the halt function: Fortunately, Linux
didn't miss it so far. Let's update it for the cases where the cpa is
expected to be valid; in some cases, the cpa is 'unpredictable', so we
leave it untouched.

Reviewed-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
