When sun6i_video_remote_subdev() returns NULL to subdev, no error return
code of sun6i_video_start_streaming() is assigned.
To fix this bug, ret is assigned with -EINVAL in this case.

Reported-by: TOTE Robot <oslab@tsinghua.edu.cn>
Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
Fixes: 5cc7522d ("media: sun6i: Add support for Allwinner CSI V3s")
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>