iio: dac: ad5686: fix struct name in doc comment

Fix a copy/paste mistake in the struct ad5686_state doc comment.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20241120-iio-regulator-cleanup-round-6-v1-2-d5a5360f7ec3@baylibre.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>