ASoC: da732x: Mark DC offset control registers volatile

The driver reads from the DC offset control registers during callibration
but since the registers are marked as volatile and there is a register
cache the values will not be read from the hardware after the first reading
rendering the callibration ineffective.

It appears that the driver was originally written for the ASoC level
register I/O code but converted to regmap prior to merge and this issue
was missed during the conversion as the framework level volatile register
functionality was not being used.

Signed-off-by: Mark Brown <broonie@linaro.org>
Acked-by: Adam Thomson <Adam.Thomson.Opensource@diasemi.com>
Cc: stable@vger.kernel.org