ASoC: pcm: Fix DPCM for aux_devs

When we instantiate an aux_dev we use a fake rtd as part of the process
which doesn't have a dai_link associated with it. Fix the dpcm startup
code to cope with this.
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Liam Girdwood <lrg@ti.com>

sound/soc/soc-pcm.c

@@ -2332,6 +2332,9 @@ static const struct file_operations dpcm_state_fops = {
 
 int soc_dpcm_debugfs_add(struct snd_soc_pcm_runtime *rtd)
 {
+	if (!rtd->dai_link)
+		return 0;
+
 	rtd->debugfs_dpcm_root = debugfs_create_dir(rtd->dai_link->name,
 			rtd->card->debugfs_card_root);
 	if (!rtd->debugfs_dpcm_root) {