firewire: Fix dualbuffer iso receive mode and drop buffer fill mode.

The dualbuffer DMA setup did not account for the iso trailer word
and thus didn't  work correctly.  With this fixed we can drop the
dual buffer fallback mode.

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