No need to BUG_ON() on the cache maintainance ops - they are no-ops
by default, and there is nothing in the DMA API contract that prohibits
calling them on sbus devices (even if such drivers are unlikely to
ever appear).

Similarly a dma_supported method that always returns 0 is rather
pointless.  The only thing that indicates is that no one ever calls
the method on sbus devices.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: David S. Miller <davem@davemloft.net>