firewire: fw-sbp2: skip unnecessary logout

Don't attempt to send a logout ORB if the target was already unplugged
or had its link switched off.  If two targets are attached, this
enhances the chance to quickly reconnect to the remaining target when
one target is plugged out.

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