ia64: make dma_cache_sync a no-op

ia64 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: Christoph Hellwig <hch@lst.de>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>

arch/ia64/include/asm/dma-mapping.h

@@ -48,11 +48,6 @@ static inline void
 dma_cache_sync (struct device *dev, void *vaddr, size_t size,
 	enum dma_data_direction dir)
 {
-	/*
-	 * IA-64 is cache-coherent, so this is mostly a no-op.  However, we do need to
-	 * ensure that dma_cache_sync() enforces order, hence the mb().
-	 */
-	mb();
 }
 
 #endif /* _ASM_IA64_DMA_MAPPING_H */