The Sagem/Plusnet F@ST2704N has a red label in ethernet port 4. Its purpose is
to be used as Fibre/WAN with the stock firmware.
Configure the Eth4 as WAN.
Fixes: fbbb977772 (brcm63xx: Tune the network configuration for several
routers)
Signed-off-by: Daniel González Cabanelas <dgcbueu@gmail.com>