[media] uvcvideo: Move __uvc_ctrl_get() up

This avoids the need for doing a forward declaration of __uvc_ctrl_get
(which is a static function) in later patches in this series.

Note to reviewers this patch does not change a single line of code, it
just moves the function up in uvc_ctrl.c a bit.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>