iio: adc: sun4i-gpadc: use of_device_get_match_data

The usage of of_device_get_match_data reduce the code size a bit.
Furthermore, it prevents an improbable dereference when
of_match_device() return NULL.

Signed-off-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>