soundwire: intel: prevent possible dereference in hw_params

This should not happen in production systems but we should test for
all callback arguments before invoking the config_stream callback.

Update the prototype to clarify that the first argument is mandatory.

Also use local variable instead of multiple dereferences to improve
readability.
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20190806005522.22642-2-pierre-louis.bossart@linux.intel.comSigned-off-by: Vinod Koul <vkoul@kernel.org>

drivers/soundwire/intel.c

@@ -397,8 +397,10 @@ static int intel_config_stream(struct sdw_intel *sdw,
 			       struct snd_soc_dai *dai,
 			       struct snd_pcm_hw_params *hw_params, int link_id)
 {
-	if (sdw->res->ops && sdw->res->ops->config_stream)
-		return sdw->res->ops->config_stream(sdw->res->arg,
+	struct sdw_intel_link_res *res = sdw->res;
+
+	if (res->ops && res->ops->config_stream && res->arg)
+		return res->ops->config_stream(res->arg,
 				substream, dai, hw_params, link_id);
 
 	return -EIO;

include/linux/soundwire/sdw_intel.h

@@ -8,6 +8,7 @@
  * struct sdw_intel_ops: Intel audio driver callback ops
  *
  * @config_stream: configure the stream with the hw_params
+ * the first argument containing the context is mandatory
  */
 struct sdw_intel_ops {
 	int (*config_stream)(void *arg, void *substream,