net: dsa: bcm_sf2: Ensure correct sub-node is parsed

When the bcm_sf2 was converted into a proper platform device driver and
used the new dsa_register_switch() interface, we would still be parsing
the legacy DSA node that contained all the port information since the
platform firmware has intentionally maintained backward and forward
compatibility to client programs. Ensure that we do parse the correct
node, which is "ports" per the revised DSA binding.

Fixes: d9338023

 ("net: dsa: bcm_sf2: Make it a real platform device driver")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Vivien Didelot <vivien.didelot@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>