ASoC: dapm: Change prototype of soc_widget_read

soc_widget_read API returns the register data and it is possible
that a register can contain 0xffffffff. Thus, change the prototype
of soc_widget_read to return only the error code and pass the reg
data through pointer argument.

Signed-off-by: Arun Shamanna Lakshmi <aruns@nvidia.com>
Signed-off-by: Mark Brown <broonie@linaro.org>