firewire: Move async transmit to use the general context code.

The old async transmit context handling was starting and stopping
DMA for every packet transmission.  This could cause silently failing
packet transmission, if the DMA was reprogrammed too close to being
stopped.

The general context code keeps DMA running at all times and fixes this
problem.  It's also a nice cleanup.

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