ltq-deu: fix 6.12 kernel build warnings on lantiq/xrx200

ltq-deu: fix 6.12 kernel build warnings on lantiq/xrx200

This patch fixes various missing-prototypes build warnings by:

* Mark some functions as static.
* Add missing prototypes.

Signed-off-by: Shiji Yang <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/18744
Signed-off-by: Robert Marko <[email protected]>
(cherry picked from commit d81f8e159a338318c690a1b9720a33544cfa612d)
Signed-off-by: Nora Matthias Schiffer <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/20440
Signed-off-by: Hauke Mehrtens <[email protected]>