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.com


Signed-off-by: Vinod Koul <vkoul@kernel.org>