firewire: core: fix iso context shutdown on card removal

If isochronous contexts existed when firewire-ohci was unloaded, the
core iso shutdown functions crashed with NULL dereferences, and buffers
etc. weren't released.

How the fix works:  We first copy the card driver's iso shutdown hooks
into the dummy driver, then fw_destroy_nodes notifies upper layers of
devices going away, these should shut down (including their iso
contexts), wait_for_completion(&card->done) will be triggered after
upper layers gave up all fw_device references, after which the card
driver's shutdown proceeds.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>