firewire: potentially invalid pointers used in fw_card_bm_work

The bus management workqueue job was in danger to dereference NULL
pointers.  Also, after having temporarily lifted card->lock, a few node
pointers and a device pointer may have become invalid.

Add NULL pointer checks and get the necessary references.  Also, move
card->local_node out of fw_card_bm_work's sight during shutdown of the
card.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Signed-off-by: Jarod Wilson <jwilson@redhat.com>