staging:iio:ad7606: fix voltage scales

staging:iio:ad7606: fix voltage scales

Fixes commit 17be2a2905a6ec9aa27cd59521495e2f490d2af0 ("staging: iio:
ad7606: replace range/range_available with corresponding scale").

The AD7606 devices don't have a 2.5V voltage range, they have 5V & 10V
voltage range, which is selectable via the `gpio_range` descriptor.

The scales also seem to have been miscomputed, because when they were
applied to the raw values, the results differ from the expected values.
After checking the ADC transfer function in the datasheet, these were
re-computed.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>