firewire: Don't touch DMA descriptors after appending.

When a DMA descriptor is appended to the context we sync it for
DMA and the device might potentially read it immediately.  So,
we can't set the IRQ bits in the descriptor after appending.

Signed-off-by: Kristian Høgsberg <krh@redhat.com>
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>