vfio-ccw: make convert_ccw0_to_ccw1 static

Reported by sparse.

Fixes: 7f8e89a8 ("vfio-ccw: Factor out the ccw0-to-ccw1 transition")
Signed-off-by: Cornelia Huck <cohuck@redhat.com>
Message-Id: <20190624090721.16241-1-cohuck@redhat.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>