[POWERPC] Fix initialization and usage of dma_mask

powerpc has a couple of bugs in the usage of dma_masks that tend to
break when drivers explicitly try to set a 32-bit mask for example.

First, the code that generates the pci devices from the OF device-tree
doesn't initialize the mask properly, then our implementation of
set_dma_mask() was trying to validate the -previous- mask value, not the
one passed in as an argument.

This fixes these problems.

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