s390x: css: Store CSS Characteristics

CSS characteristics exposes the features of the Channel SubSystem.
Let's use Store Channel Subsystem Characteristics to retrieve
the features of the CSS.

Signed-off-by: Pierre Morel <pmorel@linux.ibm.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Acked-by: Janosch Frank <frankja@linux.ibm.com>
Link: https://lore.kernel.org/kvm/1615545714-13747-2-git-send-email-pmorel@linux.ibm.com/


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