[RFC PATCH powerpc] Fix a dma_mask issue of vio

I encountered following issue:
[    0.283035] ibmvscsi 30000015: couldn't initialize event pool
[    5.688822] ibmvscsi: probe of 30000015 failed with error -1

which prevents the storage from being recognized, and the machine from
booting.

After some digging, it seems that it is caused by commit 4886c399da

as dma_mask pointer in viodev->dev is not set, so in
dma_set_mask_and_coherent(), dma_set_coherent_mask() is not called
because dma_set_mask(), which is dma_set_mask_pSeriesLP() returned EIO.
While before the commit, dma_set_coherent_mask() is always called. 

I tried to replace dma_set_mask_and_coherent() with
dma_coerce_mask_and_coherent(), and the machine could boot again. 

But I'm not sure whether this is the correct fix...

---
 arch/powerpc/kernel/vio.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/arch/powerpc/kernel/vio.c b/arch/powerpc/kernel/vio.c
index e7d0c88..76a6482 100644
--- a/arch/powerpc/kernel/vio.c
+++ b/arch/powerpc/kernel/vio.c

@@ -1419,7 +1419,7 @@ struct vio_dev *vio_register_device_node(struct device_node *of_node)
 
 		/* needed to ensure proper operation of coherent allocations
 		 * later, in case driver doesn't set it explicitly */
-		dma_set_mask_and_coherent(&viodev->dev, DMA_BIT_MASK(64));
+		dma_coerce_mask_and_coherent(&viodev->dev, DMA_BIT_MASK(64));
 	}
 
 	/* register with generic device framework */