ltq-atm: fix build warnings on lantiq/xway

ltq-atm: fix build warnings on lantiq/xway

Mark some functions as static to fix various missing-prototypes
build warnings on 6.12 kernel.

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 ac18f03bd97c85ece73503115f2b9ac7de3b63e3)
Signed-off-by: Nora Matthias Schiffer <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/20440
Signed-off-by: Hauke Mehrtens <[email protected]>