powerpc/dma: properly wire up the unmap_page and unmap_sg methods

The unmap methods need to transfer memory ownership back from the
device to the cpu by identical means as dma_sync_*_to_cpu. I'm not
sure powerpc needs to do any work in this transfer direction, but
given that it does invalidate the caches in dma_sync_*_to_cpu already
we should make sure we also do so on unmapping.

Signed-off-by: Christoph Hellwig <hch@lst.de>
[mpe: s/dir/direction in dma_nommu_unmap_page()]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>