ASoC: soc-dai: clarify return value for get_sdw_stream()

Previous changes move to use ERR_PTR(-ENOTSUPP), but it's not clear
what implementations can return in case of errors. Explicitly document
that NULL is not a possible return value, only ERR_PTR with a negative
error code is valid.

Fixes: 308811a3 ('ASoC: soc-dai: return proper error for get_sdw_stream()')
Cc: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Reported-by: Bard Liao <yung-chuan.liao@linux.intel.com>
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Reviewed-by: Rander Wang <rander.wang@linux.intel.com>
Signed-off-by: Bard Liao <yung-chuan.liao@linux.intel.com>
Link: https://lore.kernel.org/r/20200904182854.3944-2-yung-chuan.liao@linux.intel.comSigned-off-by: Mark Brown <broonie@kernel.org>

include/sound/soc-dai.h

@@ -471,7 +471,8 @@ static inline int snd_soc_dai_set_sdw_stream(struct snd_soc_dai *dai,
  * This routine only retrieves that was previously configured
  * with snd_soc_dai_get_sdw_stream()
  *
- * Returns pointer to stream or -ENOTSUPP if callback is not supported;
+ * Returns pointer to stream or an ERR_PTR value, e.g.
+ * ERR_PTR(-ENOTSUPP) if callback is not supported;
  */
 static inline void *snd_soc_dai_get_sdw_stream(struct snd_soc_dai *dai,
 					       int direction)