dsa: lan9303: Port 0 is xMII port

In preparing to move the adjust_link logic into the phylink_mac_link_up
api, change the macro used to check for the cpu port. In
phylink_mac_link_up, the phydev pointer passed in for the CPU port is
NULL, so we can't keep using phy_is_pseudo_fixed_link(phydev).

Signed-off-by: Jerry Ray <jerry.ray@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>