net: greth: convert to hw_features

Note: Driver modifies its struct net_device_ops. This will break if used for
multiple devices that are not all the same (if that HW config is possible).

Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>