iio: mtk-auxadc: fix case IIO_CHAN_INFO_PROCESSED

The previous driver does't apply the necessary scaling to take the
voltage range into account.
We change readback value from raw data to input voltage to fix case
IIO_CHAN_INFO_PROCESSED.

Fixes: ace4cdfe ("iio: adc: mt2701: Add Mediatek auxadc driver for mt2701.")
Signed-off-by: Hui Liu <hui.liu@mediatek.com>
Link: https://lore.kernel.org/r/20210926073028.11045-2-hui.liu@mediatek.com


Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>