s390x/css: Use static initialization for channel_subsys fields

machine_init() will be gone, but we don't need it if we just
initialize the channel_subsys fields statically.

Cc: Cornelia Huck <cornelia.huck@de.ibm.com>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Richard Henderson <rth@twiddle.net>
Cc: Alexander Graf <agraf@suse.de>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <1455656347-29033-4-git-send-email-ehabkost@redhat.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
[adapted on top of indicator changes]
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
