ASoC: adau1701: switch to direct regmap API usage

The hardware I/O has to be open-coded due to registers of unequal sizes.
Other than that, the transition is straight forward.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@linaro.org>