[SPARC64]: Fix OOPS in dma_sync_*_for_device()

I included these operations vector cases for situations
where we never need to do anything, the entries aren't
filled in by any implementation, so we OOPS trying to
invoke NULL pointer functions.

Really make them NOPs, to fix the bug.

Signed-off-by: David S. Miller <davem@davemloft.net>