s390x: rename sclp_setup() to sclp_ascii_setup()

We'll be introducing some other sclp related files including setup
functions soon. Also allow to call sclp_service_call() from these
(including reusing the buffer).

Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20180213162321.20522-6-david@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>