[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>