firewire: fw-sbp2: use device generation, not card generation

There was a small window where a login or reconnect job could use an
already updated card generation with an outdated node ID.  We have to
use the fw_device.generation here, not the fw_card.generation, because
the generation must never be newer than the node ID when we emit a
transaction.  This cannot be guaranteed with fw_card.generation.

Furthermore, the target's and initiator's node IDs can be obtained from
fw_device and fw_card.  Dereferencing their underlying topology objects
is not necessary.

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

Verified in concert with subsequent memory barriers patch to fix 'giving
up on config rom' issues on multiple system and drive combinations that
were previously affected.

Signed-off-by: Jarod Wilson <jwilson@redhat.com>