target: add support for START_STOP_UNIT SCSI opcode

AIX servers using VIOS servers that virtualize FC cards will have a
problem booting without support for START_STOP_UNIT.

Cite sbc3r36 exactly, clean up if conditions (rob + hch)

Signed-off-by: Brian Bunker <brian@purestorage.com>
Signed-off-by: Spencer Baugh <sbaugh@catern.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: "Robert Elliott (Server Storage)" <Elliott@hp.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>