scsi: sd: optionally set power condition in START STOP UNIT

Adds a new scsi_device flag, start_stop_pwr_cond:  If enabled, the sd
driver will not send plain START STOP UNIT commands but ones with the
power condition field set to 3 (standby) or 1 (active) respectively.

Some FireWire disk firmwares do not stop the motor if power condition is
zero.  Or worse, they become unresponsive after a START STOP UNIT with
power condition = 0 and start = 0.

http://lkml.org/lkml/2008/4/29/704

This patch only adds the necessary code to sd_mod but doesn't activate
it.  Follow-up patches to the FireWire drivers will add detection of
affected devices and enable the code for them.

I did not add power condition values to scsi_error.c::scsi_eh_try_stu()
for now.  The three firmwares which suffer from above mentioned problems
do not need START STOP UNIT in the error handler, and they are not
adversely affected by START STOP UNIT with power condition = 0 and start
= 1 (like scsi_eh_try_stu() sends it if scsi_device.allow_restart is
enabled).

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