ASoC: cs42l51: Convert to direct regmap API usage

As part of phasing out the ASoC level register I/O code (which is now just
a thin wrapper around regmap anyway) convert the cs42l51 driver to use the
regmap API directly.

We now no longer initialise the cache from hardware at startup, the regmap
caches are smart enough to understand which registers are actually cached
and read on demand. This should have no visible effect on the system.

Signed-off-by: Mark Brown <broonie@linaro.org>
Acked-by: Brian Austin <brian.austin@cirrus.com>