iio: dac: ad3552r: uniform structure names

Use same driver file name (ad3552r) for structure names used
for all variants.

Signed-off-by: Angelo Dureghello <adureghello@baylibre.com>
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20240522150141.1776196-7-adureghello@baylibre.org


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