[ALSA] ASoC at91 - Fix NULL pointer dereference in at91_i2s_shutdown

This patch fixes a NULL pointer exception which occurs when a
substream is opened and immediately closed.

Signed-off-by: Frank Mandarino <fmandarino@endrelia.com>
Signed-off-by: Liam Girdwood <lg@opensource.wolfsonmicro.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>

sound/soc/at91/at91-i2s.c

@@ -296,6 +296,13 @@ static int at91_i2s_startup(struct snd_pcm_substream *substream)
 	ssc_p->dir_mask |= dir_mask;
 	spin_unlock_irq(&ssc_p->lock);
 
+	/*
+	 * dma_data is not set until hw_params() is called and
+	 * shutdown() depends on this value being NULL if hw_params()
+	 * was not called.
+	 */
+	rtd->cpu_dai->dma_data = NULL;
+
 	return 0;
 }