mtk-jpegdec: add stop cmd interface for jpgdec

Add stop cmd interface for jpgdec to stop stream

Signed-off-by: kyrie wu <kyrie.wu@mediatek.com>
Signed-off-by: irui wang <irui.wang@mediatek.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>