staging: iio: tsl2x7x: move power and diode settings into header file

The power and diode defines are needed for the platform data so this
patch moves the defines out of the .c file and into the header file. A
comment for the diode is also cleaned up while this code is touched.

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