firewire: fw-sbp2: fix spindown for PL-3507 and TSB42AA9 firmwares

Reported by Tino Keitel:  PL-3507 with firmware from Prolific does not
spin down the disk on START STOP UNIT with power condition = 0 and start
= 0.  It does however work with power condition = 2 or 3.

Also found while investigating this:  DViCO Momobay CX-1 and FX-3A (TI
TSB42AA9/A based) become unresponsive after START STOP UNIT with power
condition = 0 and start = 0.  They stay responsive if power condition is
set when stopping the motor.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Tested-by: Tino Keitel <tino.keitel@gmx.de>