spi: sh-msiof: Extract sh_msiof_spi_{start,stop}() helpers

Based on an old patch by Guennadi Liakhovetski <g.liakhovetski@gmx.de>

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Mark Brown <broonie@linaro.org>