[media] V4L menu: move USB drivers section to the top

USB webcams are some of the most used V4L devices, so move it to a more
prominent place in the menu instead of being at the end.

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