ASoC: cx20442: fix regression by adding back .read/.write

commit 39b5a0f8 ("ASoC: cx20442: don't use reg_cache")
removed .read/.write from driver, but it might breaks non-regmap
driver, because ALSA SoC framework might call it.

To fix this regression, this patch back .read/.write.
and also this patch uses cx20442 internal reg_cache
which is needed for .read/.write.

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