[media] s5p-mfc-dec: Don't use encoder stop command

The decoder should handle V4L2_DEC_CMD_STOP to trigger drain,
but it currently expecting V4L2_ENC_CMD_STOP.

Signed-off-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Acked-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>