firewire: ohci: use memory barriers to order descriptor updates

firewire: ohci: use memory barriers to order descriptor updates

When we append to a DMA program, we need to ensure that the order in
which initialization of the new descriptors and update of the
branch_address of the old tail descriptor, as seen by the PCI device,
happen as intended.

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