ASoC: sun4i-i2s: Rework MCLK divider calculation

The MCLK divider calculation is currently computing the ideal divider using
the oversample rate, the sample rate and the parent rate.

However, since we have access to the frequency is supposed to be running at
already, and as it turns out we're using it to compute the oversample rate,
we can just use the ratio between the parent rate and the MCLK rate to
simplify a bit the formula.

Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://lore.kernel.org/r/dcc5deb2eb650758d268bddd20f60ba58856d024.1566242458.git-series.maxime.ripard@bootlin.com


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