iio: dac: ad3552r: extract common code (no changes in behavior intended)

[ Upstream commit f665d7d3 ]

Extracting common code, to share common code to be used later
by the AXI driver version (ad3552r-axi.c).

Signed-off-by: Angelo Dureghello <adureghello@baylibre.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20241028-wip-bl-ad3552r-axi-v0-iio-testing-v9-6-f6960b4f9719@kernel-space.org


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>