libata: fix translation for START STOP UNIT

libata's SCSI translation for the SCSI START STOP UNIT command with the
START bit clear (i.e.  stopping the drive) appears to be incorrect.  It
sends an ATA STANDBY command with the time period set to 0, which the code
comment says means "now", but the ATA standard says this means disable the
standby timer, which effectively does nothing.  Change this to issue a
STANDBY IMMEDIATE command which will actually spin the drive down.  The SAT
(SCSI/ATA Translation) standard revision 9 concurs with this choice.

Signed-off-by: Robert Hancock <hancockr@shaw.ca>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>