staging: iio: tsl2x7x: use GPL-2.0+ SPDX license identifier

The summary text for the GPL is not needed since the SPDX identifier
is a legally binding shorthand that can be used instead.

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