staging: iio: light: isl29018: Use standard sysfs attributes for scale and integration time

This patch refactors the isl29018 driver code in order to use standard
sysfs attributes for scale and integration time.

ISL29018 light sensor uses four ranges and four ADC's resolutions
which influence the calculated lux. Adc resolution is strongly
connected to integration time and range should be controlled by scale.

This patch introduces the usage of integration time and scale instead
of adc resolution and range.

Signed-off-by: Roberta Dobrescu <roberta.dobrescu@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>