netdev: ethtool RXHASH flag

This adds ethtool and device feature flag to allow control
of receive hashing offload.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Acked-by: Jeff Garzik <jgarzik@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>