s390x: css: extending the subchannel modifying functions

To enable or disable measurement we will need specific
modifications on the subchannel.

Signed-off-by: Pierre Morel <pmorel@linux.ibm.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Link: https://lore.kernel.org/kvm/1615545714-13747-4-git-send-email-pmorel@linux.ibm.com/


Signed-off-by: Janosch Frank <frankja@linux.ibm.com>