firewire: Future proof the iso ioctls by adding a handle for the iso context.

Currently create context always returns 0 and the other iso
ioctls will expect user space to pass that in for subsequent ioctls.

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