iio: mtk-auxadc: fix case IIO_CHAN_INFO_PROCESSED

iio: mtk-auxadc: fix case IIO_CHAN_INFO_PROCESSED

commit c0b85c0328514a4cdbcc882e9c38cd6d48b7f3bd 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: 8c5d2bdf2619 ("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>