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.c b/sound/soc/fsl/mpc5200_dma.c
index efec33a..f0a2d40 100644
--- a/sound/soc/fsl/mpc5200_dma.c
+++ b/sound/soc/fsl/mpc5200_dma.c
@@ -456,6 +456,7 @@
 		return -ENODEV;
 
 	spin_lock_init(&psc_dma->lock);
+	mutex_init(&psc_dma->mutex);
 	psc_dma->id = be32_to_cpu(*prop);
 	psc_dma->irq = irq;
 	psc_dma->psc_regs = regs;