spi: sh-msiof: Convert to let spi core validate xfer->bits_per_word

Set bits_per_word_mask so the spi core will reject transfers that attempt
to use an unsupported bits_per_word value.

Signed-off-by: Geert Uytterhoeven <geert+renesas@linux-m68k.org>
Acked-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Mark Brown <broonie@linaro.org>