[media] v4l2-common: move v4l2_ctrl_check to cx2341x

The v4l2_ctrl_check() helper function is now only used in cx2341x.
Move it there and make it static.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>