ASoC: ak4642: fixup cache register table

ak4642 register was 8bit, but cache table was defined as 16bit.
ak4642 doesn't work correctry without this patch.

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