iio: ti-ads8688: use of_device_id for OF matching

iio: ti-ads8688: use of_device_id for OF matching

The of_device_id was added to allow module autoloading, but it should be
also used to allow driver matching via Devicetree.

This also fixes W=1 warning:
  drivers/iio/adc/ti-ads8688.c:501:34: error: ‘ads8688_of_match’ defined but not used [-Werror=unused-const-variable=]

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220501103447.111392-2-krzysztof.kozlowski@linaro.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>