iio: light: vl6180: Move range check to integration time setter, cleanup

This improves code uniformity (range checks for als_gain are also done
in the setter). Also unmangle rounding and calculation of register value.

The calculated integration time it_ms is required in the next patch of
the series.

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