powerpc/pasemi: Fix coherent_dma_mask for dma engine

Commit 817820b0 ("powerpc/iommu: Support "hybrid" iommu/direct DMA
ops for coherent_mask < dma_mask) adds a check of coherent_dma_mask for
dma allocations.

Unfortunately current PASemi code does not set this value for the DMA
engine, which ends up with the default value of 0xffffffff, the result
is on a PASemi system with >2Gb ram and iommu enabled the the onboard
ethernet stops working due to an inability to allocate memory. Add an
initialisation to pci_dma_dev_setup_pasemi().

Signed-off-by: Darren Stevens <darren@stevens-zone.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>