firewire: fw-sbp2: let SCSI shutdown commands through before logout

This affects of course only the "soft shutdown" case, e.g. "modprobe -r
firewire-sbp2", while it doesn't matter for hot unplug.

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