MMC-5 6.40.2.6 specifies that START STOP UNIT succeeds when the drive
already has the requested state.  cmd_start_stop_unit() fails when
asked to eject while the tray is open and locked.  Fix that.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>