media: cx231xx: set device_caps for 417

The video_device for the MPEG encoder did not set device_caps.

Add this, otherwise the video device can't be registered (you get a
WARN_ON instead).

Not seen before since currently 417 support is disabled, but I found
this while experimenting with it.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>