media: sun6i-csi: add a missing return code

As pointed by smatch, there's a missing return code:

	drivers/media/platform/sunxi/sun6i-csi/sun6i_video.c:485 sun6i_video_open() warn: missing error code 'ret'

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>