ASoC: SOF: pcm: Clear the susbstream pointer to NULL on close

The spcm->stream[substream->stream].substream is set during open and was
left untouched. After the first PCM stream it will never be NULL and we
have code which checks for substream NULLity as indication if the stream is
active or not.
For the compressed cstream pointer the same has been done, this change will
correct the handling of PCM streams.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@linux.intel.com>

sound/soc/sof/pcm.c

@@ -511,6 +511,8 @@ static int sof_pcm_close(struct snd_soc_component *component,
 		 */
 	}
 
+	spcm->stream[substream->stream].substream = NULL;
+
 	return 0;
 }