iio: adc: rn5t618-adc: use devm_iio_map_array_register() function

iio: adc: rn5t618-adc: use devm_iio_map_array_register() function

This driver already hooks a similar unwind callback via
devm_add_action_or_reset().

They pretty much do the same thing, so this change converts it to the
devm_iio_map_array_register().

Signed-off-by: Alexandru Ardelean <aardelean@deviqon.com>
Reviewed-by: Andreas Kemnade <andreas@kemnade.info>
Link: https://lore.kernel.org/r/20210926162859.3567685-1-aardelean@deviqon.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>