iio:gp2ap020a00f: Switch to new event config interface

Switch the gp2ap020a00f 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: Jacek Anaszewski <j.anaszewski@samsung.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>