powerpc: Revert c6102609 and replace it with the correct fix for vio dma mask setting

This patch reverts my previous "fix", and replace it with the correct
fix from Russell.

And as Russell pointed out -- dma_set_mask_and_coherent() (and the other
dma_set_mask() functions) are really supposed to be used by drivers
only.
Signed-off-by: NLi Zhong <zhong@linux.vnet.ibm.com>
Signed-off-by: NBenjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/vio.c

@@ -1432,7 +1432,8 @@ 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_coerce_mask_and_coherent(&viodev->dev, DMA_BIT_MASK(64));
+		viodev->dev.coherent_dma_mask = DMA_BIT_MASK(64);
+		viodev->dev.dma_mask = &viodev->dev.coherent_dma_mask;
 	}
 
 	/* register with generic device framework */