iio: dac: ad5504: fix wrong part number in kernel-doc structure name.

Probably a bit of cut and paste where someone forgot to change the
part number.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>
Link: https://lore.kernel.org/r/20210314164655.408461-5-jic23@kernel.org