net: ethernet: xgbe: expand PHY_GBIT_FEAUTRES

The macro PHY_GBIT_FEAUTRES needs to change into a bitmap in order to
support link_modes. Remove its use from xgde by replacing it with its
definition.

Probably, the current behavior is wrong. It probably should be
ANDing not assigning.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>