spi: sh-msiof: Simplify BRG's Division Ratio

As FIELD_PREP() masks the value to be stored in the field, the Baud Rate
Generator's Division Ratio handling can be simplified from a look-up
table to a single subtraction.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://patch.msgid.link/e736221942b0381fb53dc64109a1389f7ec5f44a.1747401908.git.geert+renesas@glider.be


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