staging: iio: tsl2x7x: split out als and prox persistence settings

staging: iio: tsl2x7x: split out als and prox persistence settings

The struct tsl2x7x_settings contained a persistence member that
contained both the ALS and proximity persistence fields. This patch
splits this out into two separate fields so that the bitmasks in
several parts of the code are no longer necessary.

The default persistence settings are also changed by this patch from:

- Proximity: 0 (Every proximity cycle generates an interrupt)
- ALS: 255 (60 consecutive values out of range)

to something a little more reasonable based on my testing:

- Proximity: 1 (1 proximity value out of range)
- ALS: 1 (1 value outside of threshold range)

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