MAC on the GBit versions supports 1000/Full only, however the PHY
partially claims to support 1000/Half. So let's explicitly remove
this mode.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>