[media] control.rst: improve the queryctrl code examples

The code examples on how to enumerate controls were really long in the
tooth. Update them.

Using FLAG_NEXT_CTRL is preferred these days, so give that example first.

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