openrisc: remove the sync_single_for_cpu DMA operation
openrisc does all the required cache maintainance at dma map time, and none
at unmap time. It thus has to implement sync_single_for_device to match
the map cace for buffer reuse, but there is no point in doing another
invalidation in the sync_single_cpu_case, which in terms of cache
maintainance is equivalent to the unmap case.
Signed-off-by: Christoph Hellwig <hch@lst.de> Signed-off-by: Stafford Horne <shorne@gmail.com>