iio: adc: at91-sama5d2_adc: move the check of oversampling in its function

iio: adc: at91-sama5d2_adc: move the check of oversampling in its function

Oversampling values are checked anyway in at91_adc_emr_config(). Remove
the checking of these from at91_adc_write_raw() and return -EINVAL
instead in at91_adc_emr_config().

Suggested-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lore.kernel.org/r/20220803102855.2191070-8-claudiu.beznea@microchip.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>