ide/atapi: Fix START STOP UNIT command completion

The command must be completed on all code paths. START STOP UNIT with
pwrcnd set should succeed without doing anything.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
(cherry picked from commit 03441c3a)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>