ASoC: rl6231: Simplify DMIC divider calculation expression

Existing implementation checks all divider values and tracks
'red' proximity value for the frequency.

But as divider array is monotonically increasing the first
divider that gives DMIC rate in 3MHz range is the best one
we should use. No need for 'red' zone tracking.

Additionally make sure that DMIC frequency is higher 1MHz.

Signed-off-by: Anatol Pomozov <anatol.pomozov@gmail.com>
Acked-by: Oder Chiou <oder_chiou@realtek.com>
Signed-off-by: Mark Brown <broonie@kernel.org>