ASoC: soc-dai: return proper error for get_sdw_stream()

snd_soc_dai_get_sdw_stream() returns null if dai does not support
this callback, this is no very useful for the caller to
differentiate if this is an error or unsupported call for the dai.

return -ENOTSUPP in cases where this callback is not supported.
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Link: https://lore.kernel.org/r/20200316151110.2580-1-srinivas.kandagatla@linaro.orgSigned-off-by: Mark Brown <broonie@kernel.org>

include/sound/soc-dai.h

@@ -433,7 +433,7 @@ 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 NULL;
+ * Returns pointer to stream or -ENOTSUPP if callback is not supported;
  */
 static inline void *snd_soc_dai_get_sdw_stream(struct snd_soc_dai *dai,
 					       int direction)
@@ -441,7 +441,7 @@ static inline void *snd_soc_dai_get_sdw_stream(struct snd_soc_dai *dai,
 	if (dai->driver->ops->get_sdw_stream)
 		return dai->driver->ops->get_sdw_stream(dai, direction);
 	else
-		return NULL;
+		return ERR_PTR(-ENOTSUPP);
 }
 
 #endif