[POWERPC] Fix potential cell IOMMU bug when switching back to default DMA ops

If we get a 64-bit dma mask we switch to the fixed ops and call
cell_dma_dev_setup().  If the driver then switches back to a 32-bit dma
mask for any reason we don't call cell_dma_dev_setup() again, which
has the potential to leave bogus data in dev->archdata.dma_data.

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