Will return -EINVAL using standard framework api when test stateless
decoder with cmd VIDIOC_(TRY)DECODER_CMD.
Using another return value to adjust v4l2 compliance test for user
driver(GStreamer/Chrome) won't use decoder cmd.
Fixes: 8cdc3794b2e3 ("media: mtk-vcodec: vdec: support stateless API")
Signed-off-by: Yunfei Dong <yunfei.dong@xxxxxxxxxxxx>