staging: iio: tsl2x7x: rename prx to prox for consistency

The driver mostly uses the 'prox' naming convention for most of the
proximity settings, however prx_time and tsl2x7x_prx_gain was present.
This patch renames these to prox_time and tsl2x7x_prox_gain for
consistency with everything else in the driver.

The kernel documentation for prx_gain is corrected to prox_gain so that
it matches what is actually in the structure.

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