net: dsa: felix: Allow PHY to AN 10/100/1000 with 2500 serdes link

If the serdes link is set to 2500 using interfce type 2500base-X, lower
link speeds over on the line side should still be supported.
Rate adaptation is done out of band, in our case using AQR PHYs this is
done using flow control.

Signed-off-by: Alex Marginean <alexandru.marginean@nxp.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>