media: uvcvideo: Refactor iterators

Avoid using the iterators after the list_for_each() constructs.
This patch should be a NOP, but makes cocci, happier:

drivers/media/usb/uvc/uvc_ctrl.c:1861:44-50: ERROR: invalid reference to the index variable of the iterator on line 1850
drivers/media/usb/uvc/uvc_ctrl.c:2195:17-23: ERROR: invalid reference to the index variable of the iterator on line 2179

Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>