ASoC: tas2770: Refactor sample rate function

Refactor the tas2770_set_samplerate to simplify the code and access the
I2C bus only once per rate request. The ramp rate and sample rate bits
are contained in the same register so a single call to the
snd_soc_update_bits function is all that is needed

Signed-off-by: Dan Murphy <dmurphy@ti.com>
Link: https://lore.kernel.org/r/20200918190548.12598-9-dmurphy@ti.com
Signed-off-by: Mark Brown <broonie@kernel.org>