spi: sh-msiof: Convert to generic unused native cs handling.

Currently the MSIOF SPI driver uses custom code to handle the unused
native chip select with GPIO chip selects.
Convert the driver to use the new generic handling in the SPI core.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20200102133822.29346-3-geert+renesas@glider.be


Signed-off-by: Mark Brown <broonie@kernel.org>