ieee1394: sbp2: spin disks down on suspend and shutdown

This instructs sd_mod to send START STOP UNIT on suspend and resume,
and on driver unbinding or unloading (including when the system is shut
down).

We don't do this though if multiple initiators may log in to the target.

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