soundwire: intel: fix NULL/ERR_PTR confusion

snd_soc_dai_get_sdw_stream() can only return the pointer to stream or
an ERR_PTR value, NULL is not a possible value.

Fixes: 09553140 ('soundwire: intel: implement get_sdw_stream() operations')
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/20200903204739.31206-3-yung-chuan.liao@linux.intel.comSigned-off-by: Vinod Koul <vkoul@kernel.org>

drivers/soundwire/intel.c

@@ -1126,7 +1126,7 @@ static void *intel_get_sdw_stream(struct snd_soc_dai *dai,
 		dma = dai->capture_dma_data;
 
 	if (!dma)
-		return NULL;
+		return ERR_PTR(-EINVAL);
 
 	return dma->stream;
 }