net: dsa: bcm_sf2: Include address 0 for MDIO diversion

We need to include MDIO address 0, which is how our Device Tree blobs
indicate where to find the external BCM53125 switches.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>