iio: light: vl6180: Cleanup als_gain lookup, avoid register readback

Instead of manually iterating the array of allowed gain values, use
find_closest. Storing the current gain setting avoids accessing the
hardware on each query.

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