V4L/DVB (5869): Add check for valid control ID to v4l2_ctrl_next.

If v4l2_ctrl_next is called without the V4L2_CTRL_FLAG_NEXT_CTRL then it 
should check whether the passed control ID is valid and return 0 if it 
isn't. Otherwise a for-loop over the control IDs will never end.

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