firewire: Generalize the iso transmit descriptor buffer logic.

The descriptor circular buffer logic used for iso transmission is
useful for async transmit too, so pull the sharable logic out in
a few standalone functions.

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