firewire: fw-sbp2: better fix for NULL pointer dereference in scsi_remove_device

Patch "firewire: fw-sbp2: fix NULL pointer deref. in scsi_remove_device"
had the unintended effect that firewire-sbp2 could not be unloaded
anymore until all SBP-2 devices were unplugged.

We now fix the NULL pointer bug by reacquiring a reference to the sdev
instead of holding a reference to the sdev (and to the module) all the
time.

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