ASoC: Intel: boards: hda_dsp_common: use NULL pointer assignment, not 0

Fix Sparse warning:
hda_dsp_common.c:66:37: warning: Using plain integer as NULL pointer
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20200102195952.9465-2-pierre-louis.bossart@linux.intel.comSigned-off-by: Mark Brown <broonie@kernel.org>

sound/soc/intel/boards/hda_dsp_common.c

@@ -63,7 +63,7 @@ int hda_dsp_hdmi_build_controls(struct snd_soc_card *card,
 				"%s: mapping HDMI converter %d to PCM %d (%p)\n",
 				__func__, i, hpcm->device, spcm);
 		} else {
-			hpcm->pcm = 0;
+			hpcm->pcm = NULL;
 			hpcm->device = SNDRV_PCM_INVALID_DEVICE;
 			dev_warn(card->dev,
 				 "%s: no PCM in topology for HDMI converter %d\n\n",