mediatek/filogic: fix Totolink X6000R sysupgrade failed

mediatek/filogic: fix Totolink X6000R sysupgrade failed

Fix Totolink X6000R image failing to upgrade via sysupgrade due to an
incorrect upgrade case. The fallback upgrade path used the NAND upgrade
routine, which caused the "layout volume not found" error on NOR flash
devices. By adding a specific case for this board, sysupgrade now uses
`default_do_upgrade`, which is compatible with the NOR flash layout.

Fixes: 7cd10ad
Signed-off-by: Ayra Hikari <ayrahikari@linuxmail.org>
(cherry picked from commit 7cd10add2de3c8227cf7383c7ca5cf3b12eeb822)
Link: https://github.com/openwrt/openwrt/pull/20674
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>