iio: potentiometer: mcp4131: use of_device_get_match_data()

Try to get the device's data using OF API. In case of failure, fallback
to the spi_get_device_id() method.

Signed-off-by: Slawomir Stepien <sst@poczta.fm>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>