firewire: Streamline userspace interface structs.

Make event struct layout common part include the closure and add a
union for all event types; provide a mechanism for setting the
bus reset event closure.

Shuffle struct fw_cdev_queue_iso fields around to be 64-bit safe.

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