powerpc: Fix dma_map_sg() cache flushing on non coherent platforms

On PowerPC 4xx or other non cache-coherent platforms, we lost the
appropriate cache flushing in dma_map_sg() when merging the 32 and
64-bit DMA code (commit 4fc665b88a79a45bae8bbf3a05563c27c7337c3d,
"powerpc: Merge 32 and 64-bit dma code").  This restores it.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Becky Bruce <beckyb@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>