ASoC: cs4271: convert to direct regmap API usage

By using the regmap API directly, we can make use of the
.write_flag_mask for SPI, which allows us to drop the strange register
hacks that were necessary so far.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Acked-by: Alexander Sverdlin <alexander.sverdlin@gmx.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>