The maximum DMIC clock rate is 3.072 MHz for most DMIC. And it will get better
performance in higher clock rate. If we set maximum to 3 MHz in driver, we will
get a clock rate which is not even close to 3 MHz.
For example, if DMIC clock source is 24.576 MHz, the DMIC clock will be about
1.5 MHz in current code. But it will be 3.072 MHz with this patch.

Signed-off-by: John Lin <john.lin@realtek.com>
Signed-off-by: Mark Brown <broonie@kernel.org>