[POWERPC] Implement pci_set_dma_mask() in terms of the dma_ops

PowerPC currently doesn't implement pci_set_dma_mask(), which means drivers
calling it will get the generic version in drivers/pci/pci.c.

The powerpc dma mapping ops include a dma_set_mask() hook, which luckily is
not implemented by anyone - so there is no bug in the fact that the hook
is currently never called.

However in future we'll add implementation(s) of dma_set_mask(), and so we
need pci_set_dma_mask() to call the hook.

To save adding a hook to the dma mapping ops, pci-set_consistent_dma_mask()
simply calls the dma_set_mask() hook and then copies the new mask into
dev.coherenet_dma_mask.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>