ASoC: sunxi: i2s: Implement set_sysclk

In our i2s driver, we were previously trying to guess which oversample the
user wanted to use by looking at the rate and trying to max it.

However, the cards, and especially simple-card with its mclk-fs property
will already provide the expected oversample ratio by using the set_sysclk
callback.

We can thus implement it and remove the logic to deal with the runtime
guess.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Mark Brown <broonie@kernel.org>