staging:iio:tsl2x7x: Switch to new event config interface

Switch the tsl2x7x driver to the new IIO event config interface as the old one
is going to be removed.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Cc: Jon Brenner <jbrenner@taosinc.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>