iio: light: vl6180: Correct ALS scale for non-default gain/integration time

The reported scale was only correct for the default settings of 100 ms
integration time and gain 1.

This aligns the reported scale with the behaviour of any other IIO driver
and the documented ABI, but may require userspace changes if someone uses
non-default settings.

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>