ASoC: stm32: dfsdm: add stm32_adfsdm_dummy_cb() callback

Adapt STM32 DFSDM driver to a change in iio_channel_get_all_cb() API.
The callback pointer becomes a requested parameter of this API,
so add a dummy callback to be given as parameter of this function.
However, the stm32_dfsdm_get_buff_cb() API is still used instead,
to optimize DMA transfers.

Signed-off-by: Olivier Moysan <olivier.moysan@st.com>
Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Acked-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20201121161457.957-2-nuno.sa@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>