ASoC: io: Retrieve val_bytes from the regmap API

Allow us to build infrastructure which needs to know the size of a value
without requiring regmap based drivers to supply this information to both
ASoC and regmap by asking regmap for the value.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Liam Girdwood <lrg@ti.com>