ASoC: adau1701: remove control_data assignment

codec->control_data has to be left unset to make the ASoC core access
the regmap properly.

That bug slipped in during a rebase session of the driver refactoring.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@linaro.org>