media: v4l2-ctrls-core.c: Do not use iterator outside loop

media: v4l2-ctrls-core.c: Do not use iterator outside loop

Simplify a bit the code introducing a new variable for iterating through
the control list.

It also makes smatch happy:

drivers/media/v4l2-core/v4l2-ctrls-api.c:1091 v4l2_query_ext_ctrl() warn: iterator used outside loop: 'ref'

Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
[hverkuil: fix tiny whitespace issue in 'pos  = ref': use just one space]