qv4l2: fix interval updating, use device_caps, handle conversion errors.

qv4l2: fix interval updating, use device_caps, handle conversion errors.

Choosing a new interval when capturing failed (of course), but the combobox
wasn't put back to the old position.

Now support device_caps for better capability reporting.

If a format can't be converted, then still memcpy it: that's better than a
black window.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>