net: dsa: microchip: really look for phy-mode in port nodes

The previous implementation failed to account for the "ports" node. The
actual port nodes are not child nodes of the switch node, but a "ports"
node sits in between.

Fixes: edecfa98

 ("net: dsa: microchip: look for phy-mode in port nodes")
Signed-off-by: Helmut Grohne <helmut.grohne@intenta.de>
Signed-off-by: David S. Miller <davem@davemloft.net>