iio: adc: stm32-dfsdm: Factor out core of reading INFO_RAW

This allows the claim on direct mode to be release in one place
rather than a lot of error paths, simplifying code.

Reviewed-by: Olivier Moysan <olivier.moysan@foss.st.com>
Link: https://patch.msgid.link/20250217141630.897334-8-jic23@kernel.org


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