ASoC: wm8996: Replace codec->control_data with wm8996->regmap

With the ongoing component-ization of the ASoC framework and the continuing
migration to using regmap for IO the control_data field of the snd_soc_codec
struct will eventually be removed. Prepare the wm8996 driver for this by using
wm8996->regmap instead of accessing the CODEC's control_data field.

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