ltq-vdsl-vr11: fix missing-prototypes build warnings

ltq-vdsl-vr11: fix missing-prototypes build warnings

* Mark some functions as static.
* Include function prototypes header file.

Signed-off-by: Shiji Yang <yangshiji66@outlook.com>
Link: https://github.com/openwrt/openwrt/pull/18744
Signed-off-by: Robert Marko <robimarko@gmail.com>
(cherry picked from commit b219c749b723821b96dc181a17240ab943744b95)
Signed-off-by: Nora Matthias Schiffer <neocturne@universe-factory.net>
Link: https://github.com/openwrt/openwrt/pull/20440
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>