staging: iio: cdc: ad7746: Push handling of supply voltage scale to userspace.

The supply voltage is attenuated by 6 before being fed to the ADC.
Handle this explicitly rather than pre-multiplying the _raw value by 6.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20220626122938.582107-5-jic23@kernel.org