powerpc: make dma_cache_sync a no-op

powerpc does not implement DMA_ATTR_NON_CONSISTENT allocations, so it
doesn't make any sense to do any work in dma_cache_sync given that it
must be a no-op when dma_alloc_attrs returns coherent memory.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NRobin Murphy <robin.murphy@arm.com>

arch/powerpc/include/asm/dma-mapping.h

@@ -144,8 +144,6 @@ static inline phys_addr_t dma_to_phys(struct device *dev, dma_addr_t daddr)
 static inline void dma_cache_sync(struct device *dev, void *vaddr, size_t size,
 		enum dma_data_direction direction)
 {
-	BUG_ON(direction == DMA_NONE);
-	__dma_sync(vaddr, size, (int)direction);
 }
 
 #endif /* __KERNEL__ */