ASoC: dmaengine_pcm: document process callback

Add missing description of process callback.

Fixes: 78648092ef ("ASoC: dmaengine_pcm: add processing support")
Signed-off-by: Olivier Moysan <olivier.moysan@st.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

include/sound/dmaengine_pcm.h

@@ -118,6 +118,8 @@ void snd_dmaengine_pcm_set_config_from_dai_data(
  *   PCM substream. Will be called from the PCM drivers hwparams callback.
  * @compat_request_channel: Callback to request a DMA channel for platforms
  *   which do not use devicetree.
+ * @process: Callback used to apply processing on samples transferred from/to
+ *   user space.
  * @compat_filter_fn: Will be used as the filter function when requesting a
  *  channel for platforms which do not use devicetree. The filter parameter
  *  will be the DAI's DMA data.