ASoC: cs42l51: manage mclk shutdown delay

A delay must be introduced before the shutdown down of the mclk,
as stated in CS42L51 datasheet. Otherwise the codec may
produce some noise after the end of DAPM power down sequence.
The delay between DAC and CLOCK_SUPPLY widgets is too short.
Add a delay in mclk shutdown request to manage the shutdown delay
explicitly. From experiments, at least 10ms delay is necessary.
Set delay to 20ms as recommended in Documentation/timers/timers-howto.rst
when using msleep().

Signed-off-by: Olivier Moysan <olivier.moysan@st.com>
Link: https://lore.kernel.org/r/20201020150109.482-1-olivier.moysan@st.com


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