firewire: fw-sbp2: correctly dereference by container_of

Replace a cast with a container_of().  As long as nobody reorders the
structure elements, they do the same thing, but container_of() is more
readable.

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