ASoC: cs42l42: Add warnings about DETECT_MODE and PLL_START

DETECT_MODE and PLL_START must be zero while HP_PDN and ADC_PDN are
both 1. If this condition is broken it can discharge FILT+ and it
can then take up to 1 second for FILT+ to recharge.

There is no workaround required for this, simply avoiding settings
and sequences that would break the requirement. The driver already
meets the requirement.

But it is not obvious from reading the code that this requirement
exists, or what is ensuring it is met. So it would not currently be
obvious to someone changing the code that there is certain special
behaviour that must be maintained.

To avoid accidental breakage in the future:

- Add comments into the register definitions to warn about this so
  that anyone changing the code around DETECT_MODE and PLL_START is
  aware of this requirement.

- Add a comment where PLL_START is written to 1 to highlight the
  requirement and why it is satisfied.

- Add a comment in cs42l42_setup_hs_type_detect() when DETECT_MODE is
  initialized.

Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Link: https://lore.kernel.org/r/20220304144015.398656-1-rf@opensource.cirrus.com


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