ASoC: ab8500-codec: Revert back to regmap

Commit ff795d61 ("ASoC: ab8500: Convert register I/O to regmap")
initially converted the ab8500 CODEC driver to use regmap rather than
legacy ASoC IO. This was reverted though in commit 63e6d43b ("ASoC:
ab8500: Revert to using custom I/O functions") since the inital conversion
was not working properly. This was presumebly because the SOC_SINGLE_XR_SX
controls, which are used by this driver, did not properly support regmap at
that point. This has since been fixed in commit 6137a5ca ("ASoC: Prepare
SOC_SINGLE_XR_SX controls for regmap"). So revert back to regmap again.

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