ieee1394: sbp2: enforce 32bit DMA mapping

In order to use OHCI-1394 physical DMA, all s/g elements, s/g tables,
ORBs, and response buffers have to reside within the first 4 GB of the
FireWire controller's physical address space.  Set the correct mask for
DMA mappings.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>