target/s390x: Prefer MO_128 definition over magic value

The MO_128 definition was not existing when commit 6d841663be8 ("s390x/tcg: Implement VECTOR GATHER ELEMENT") introduced the ES_128 definition. It was added 2 years later in commit 4b473e0c60d ("tcg: Expand MO_SIZE to 3 bits"). Since it is now available, replace the magic '4' by the more explicit 'MO_128'. Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org> Reviewed-by: Matthew Rosato <mjrosato@linux.ibm.com> Reviewed-by: Richard Henderson <richard.henderson@linaro.org> Reviewed-by: Pierrick Bouvier <pierrick.bouvier@oss.qualcomm.com> Message-ID: <20260415220358.93063-1-philmd@linaro.org> Signed-off-by: Cornelia Huck <cohuck@redhat.com>