ASoC: si476x: Fix IO setup

The si476x is a MFD device and the CODEC driver is using the regmap struct of
the parent device, hence automatic IO setup will not work and we need to
manually call snd_soc_codec_set_cache_io(). The issue was introduced commit
d6173df3 ("ASoC: si476x: Remove custom register I/O implementation")

Fixes: d6173df3

 ("ASoC: si476x: Remove custom register I/O implementation")
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@linaro.org>
Cc: stable@vger.kernel.org