iio: adc: at91-sama5d2_adc: move oversampling storage in its function

Move the storage of oversampling_ratio in at91_adc_config_emr().
This prepares for the next commits.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lore.kernel.org/r/20220803102855.2191070-13-claudiu.beznea@microchip.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>