iio: stmpe-adc: use of_device_id for OF matching

iio: stmpe-adc: 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/stmpe-adc.c:357:34: error: ‘stmpe_adc_ids’ defined but not used [-Werror=unused-const-variable=]

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220501103447.111392-1-krzysztof.kozlowski@linaro.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>