soundwire: stream: fix NULL/IS_ERR confusion

snd_soc_dai_get_sdw_stream() can only return -ENOTSUPP or the stream,
NULL is not a possible value.

Fixes: 4550569b ('soundwire: stream: add helper to startup/shutdown streams')
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/20200903204739.31206-2-yung-chuan.liao@linux.intel.comSigned-off-by: Vinod Koul <vkoul@kernel.org>

drivers/soundwire/stream.c

@@ -1911,7 +1911,7 @@ void sdw_shutdown_stream(void *sdw_substream)
 
 	sdw_stream = snd_soc_dai_get_sdw_stream(dai, substream->stream);
 
-	if (!sdw_stream) {
+	if (IS_ERR(sdw_stream)) {
 		dev_err(rtd->dev, "no stream found for DAI %s", dai->name);
 		return;
 	}