sbp2: fix S800 transfers if phys_dma is off

If sbp2 is forced to move data via ARM handler, the maximum packet size
allowed for S800 transfers exceeds ohci1394's buffer size on platforms
where PAGE_SIZE is 4096.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Signed-off-by: Ben Collins <bcollins@ubuntu.com>