ASoC: cx20442: don't use reg_cache

Codec reg_cache is legacy feature and very few driver only are using
it. But, ALSA SoC framework needs to support it.
Codec will be merged into Component in the future, so, let's remove
legacy and unused feature from framework.

cx20442 is using reg_cache but it is only 1byte, and it doesn't use
snd_soc_write/read/update_bits function which uses reg_cache.
reg_cache user is only debugfs. Let's clean reg_cache for now.
But let's keep .write function since it can be used for new additional
feature. .read will not be used, let's remove.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>