powerpc/pci: Default to dma_direct_ops for pci dma_ops

This will allow us to remove the ppc32 specific checks in get_dma_ops()
that defaults to dma_direct_ops if the archdata is NULL.  We really
should always have archdata set to something going forward.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>