mediatek: add Zyxel NWA90AX PRO model identifier

mediatek: add Zyxel NWA90AX PRO model identifier

Adds the 81E1 model identifier for the Zyxel NWA90AX Pro to allow factory flashing to these devices.

Device tree output of the latest OEM firmware upgrade
-----------------------------------------------------
/dts-v1/;

/ {
        timestamp = <0x6860779e>;
        description = [00];
        compat-models = [81 e1 ff ff ff ff ff ff ff ff];
        fw_version = "7.10(###.3)";

        images {

Device tree of the OpenWRT factory firmware after change
--------------------------------------------------------
/dts-v1/;

/ {
        timestamp = <0x68f513d9>;
        description = "Zyxel FIT (Flattened Image Tree)";
        compat-models = [80 e1 81 e1 ff ff ff ff ff ff];
        fw_version = "9.99(###.1)";
        address-cells = <0x01>;

        images {

Fixes: e34e874a11f0 ("mediatek: add support for Zyxel NWA90AX PRO")
Signed-off-by: James Davis <james.l.davis@outlook.com>
Link: https://github.com/openwrt/openwrt/pull/20759
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>