spi: sh-msiof: Update calculation of frequency dividing

sh-msiof of frequency dividing does not perform the calculation, driver have
to manage setting value in the table. It is not possible to set frequency
dividing value close to the actual data in this way. This changes from
frequency dividing of table management to setting by calculation.
This driver is able to set a value close to the actual data.

Signed-off-by: Nobuhiro Iwamatsu <nobuhiro.iwamatsu.yj@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>