iio: adc: at91_adc: use of_device_get_match_data() helper

iio: adc: at91_adc: use of_device_get_match_data() helper

This is a small tidy-up. The of_device_get_match_data() helper retrieves
the driver data from the OF table, without needed to explicitly know the
table variable (since it can retrieve it from the driver object).

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Reviewed-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20200930135048.11530-2-alexandru.ardelean@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>