frv: make dma_cache_sync a no-op

frv 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/frv/include/asm/dma-mapping.h

@@ -18,7 +18,6 @@ static inline
 void dma_cache_sync(struct device *dev, void *vaddr, size_t size,
 		    enum dma_data_direction direction)
 {
-	flush_write_buffers();
 }
 
 #endif  /* _ASM_DMA_MAPPING_H */