powerpc/macio: set a proper dma_coherent_mask

We have expected busses to set up a coherent mask to properly use the
common dma mapping code for a long time, and now that I've added a warning
macio turned out to not set one up yet.  This sets it to the same value
as the dma_mask, which seems to be what the drivers expect.

Reported-by: Mathieu Malaterre <malat@debian.org>
Tested-by: Mathieu Malaterre <malat@debian.org>
Reported-by: Meelis Roos <mroos@linux.ee>
Tested-by: Meelis Roos <mroos@linux.ee>
Signed-off-by: Christoph Hellwig <hch@lst.de>