MTU > 1500 settings are missing for WAN and LANx mt7621

[aparcar]

Dir878 /882 /882-r1 (mt7530)
OpenWrt SNAPSHOT, r18502-53b87a7a28 but seems linux 5.10 builds are afected as 5.4 kernel it's fine.

There is issue with setting MTU to 1508 for DSA driver to be rfc4638 compatibile

root@OpenWrt:# ifconfig br-lan mtu 1508
root@OpenWrt:# ifconfig eth0 mtu 1508
root@OpenWrt:~# ifconfig LAN2 mtu 1508
ifconfig: SIOCSIFMTU: Invalid argument

But even then MTU > 1500 seem to work for lan side.

root@OpenWrt:~# fping -s -M -b 1473 192.168.1.3
192.168.1.3 is alive

But to be able to use rfc4638 pppoe WAN must be set to 1508
root@OpenWrt:~# ifconfig wan mtu 1508
ifconfig: SIOCSIFMTU: Invalid argument

PPPoE session is always connecting with MTU 1492