staging: iio: tsl2x7x: convert to use read_avail

Make the sysfs attributes in_proximity0_calibscale_available, and
in_intensity0_{calibscale,integration_time}_available be created
using info_mask_separate_available on the channel configuration.

The driver assumed that the ALS increment was 2.72 ms, and the upper
range was 696 ms. Some other supported devices use 2.73 ms - 699 ms.
This patch adds support for the multiple ranges.

Signed-off-by: Brian Masney <masneyb@onstation.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>