net: dsa: felix: (re)use already existing constants

Now that there are USXGMII constants available, drop the old definitions
and reuse the generic ones.

Signed-off-by: Michael Walle <michael@walle.cc>
Tested-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>