soundwire: intel: fix NULL/ERR_PTR confusion

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: 09553140c8d7b ('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.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>