ASoC: stac9766: Remove ac97_read/ac97_write wrappers

Since the regmap conversion ac97_read/ac97_write are just simple wrappers
around snd_soc_read/snd_soc_write. Use those instead directly and remove
the wrappers.

Also use snd_soc_update_bits() where appropriate.

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