staging: iio: tsl2x7x: rename tsl2x7x_settings variable to settings

The length of the 'tsl2x7x_settings' variable within the tsl2X7X_chip
structure makes some of the line lengths greater than 80 characters for
upcoming patches. This patch shortens the name of the 'tsl2x7x_settings'
variable in this structure to just 'settings' to improve code
readability.
Signed-off-by: Brian Masney <masneyb@onstation.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>