iio: adc: meson-saradc: use of_device_get_match_data

This simplifies our _probe function by using of_device_get_match_data
instead of open-coding it. No functional changes.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>