staging: iio: tsl2x7x: rename prox_config to als_prox_config

The configuration register on the device is represented with the
prox_config member on the tsl2x7x_settings structure. According to the
TSL2772 data sheet, this register can hold: 1) the proximity drive
level, 2) ALS/Proximity long wait, and 3) the ALS gain level. This
patch renames prox_config to als_prox_config since ALS settings can
be stored here as well.

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