hwmon: iio: Use iio_read_channel_processed_scale for IIO_POWER

Instead of rescaling power channels after the fact, use the dedicated
scaling API. This should reduce any inaccuracies resulting from the
scaling.

Signed-off-by: Sean Anderson <sean.anderson@linux.dev>
Link: https://lore.kernel.org/r/20240620212005.821805-1-sean.anderson@linux.dev


Signed-off-by: Guenter Roeck <linux@roeck-us.net>