MIPS: make dma_sync_*_for_cpu a little less overzealous

When transferring DMA ownership back to the CPU there should never
be any writeback from the cache, as the buffer was owned by the
device until now.  Instead it should just be invalidated for the
mapping directions where the device could have written data.
Note that the changes rely on the fact that kmap_atomic is stubbed
out for the !HIGHMEM case to simplify the code a bit.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>