powerpc/iseries: Fix pci breakage due to bad dma_data initialization

Commit 4fc665b8 "powerpc: Merge 32 and
64-bit dma code" made changes to the PCI initialisation code that added
an assignment to archdata.dma_data but only for 32 bit code.  Commit
7eef440a "powerpc/pci: Cosmetic cleanups
of pci-common.c" removed the conditional compilation.  Unfortunately,
the iSeries code setup the archdata.dma_data before that assignment was
done - effectively overwriting the dma_data with NULL.

Fix this up by moving the iSeries setup of dma_data into a
pci_dma_dev_setup callback.

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>