ASoC: soc-dai: clarify return value for get_sdw_stream()

Previous changes move to use ERR_PTR(-ENOTSUPP), but it's not clear
what implementations can return in case of errors. Explicitly document
that NULL is not a possible return value, only ERR_PTR with a negative
error code is valid.

Fixes: 308811a3

 ('ASoC: soc-dai: return proper error for get_sdw_stream()')
Cc: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
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/20200904182854.3944-2-yung-chuan.liao@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>