iio: mtk-auxadc: fix case IIO_CHAN_INFO_PROCESSED

iio: mtk-auxadc: fix case IIO_CHAN_INFO_PROCESSED

commit c2980c64c7fd4585d684574c92d1624d44961edd upstream.

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: 8bbb5a93afe4 ("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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>