bcm63xx_enet: check 1000BASE-T advertisement configuration

If a gigabit ethernet PHY is connected to a fast ethernet MAC,
then it can detect 1000 support from the partner but not use it.

This results in a forced speed of 1000 and RX/TX failure.

Check for 1000BASE-T support and then check the advertisement
configuration before setting the MAC speed to 1000mbit.

Signed-off-by: Simon Arlott <simon@fire.lp0.eu>
Signed-off-by: David S. Miller <davem@davemloft.net>