ASoC: add locking to mpc5200-psc-ac97 driver

AC97 bus register read/write hooks need to provide locking, but the
mpc5200-psc-ac97 driver does not.  This patch adds a mutex around
the register access routines.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Jon Smirl <jonsmirl@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
diff --git a/sound/soc/fsl/mpc5200_dma.h b/sound/soc/fsl/mpc5200_dma.h
index 2000803..8d396bb 100644
--- a/sound/soc/fsl/mpc5200_dma.h
+++ b/sound/soc/fsl/mpc5200_dma.h
@@ -55,6 +55,7 @@
 	unsigned int irq;
 	struct device *dev;
 	spinlock_t lock;
+	struct mutex mutex;
 	u32 sicr;
 	uint sysclk;
 	int imr;