ASoC: stac9766: Convert to regmap

Currently the stac9766 driver still uses custom snd_soc_codec_driver IO
callbacks. This has been deprecated for a while, so convert the stac9766
driver to use regmap for its IO.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@kernel.org>