mediatek: filogic: set wan label in preinit

Implement the functionality of
target/linux/ramips/patches-5.15/700-net-ethernet-mediatek-support-net-labels.patch
in userspace, since the driver patch has been rejected as a generic solution:
https://github.com/openwrt/openwrt/pull/11435



Signed-off-by: Bjørn Mork <bjorn@mork.no>
(cherry picked from commit 1dd1ac2c356ad37366dde64b624e5eb489b19393)