ASoC: stm32: dfsdm: manage cb buffers cleanup

Ensure that resources allocated by iio_channel_get_all_cb()
are released on driver unbind.

Signed-off-by: Olivier Moysan <olivier.moysan@foss.st.com>
Link: https://lore.kernel.org/r/20221109170849.273719-1-olivier.moysan@foss.st.com


Signed-off-by: Mark Brown <broonie@kernel.org>