Some devices (e.g. Tenda AC9 based on BCM47189B0) have BCM53125 with
port 5 connected to the second Ethernet interface on the SoC. In such
case there is no PHY and we need to force link manually.
This assumes port 5 can be marked as enabled for such devices. It's not
implemented yet unfortunately.
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>