net-forcedeth: Add internal loopback support for forcedeth NICs.

Support enabling/disabling/querying internal loopback mode for
forcedeth NICs using ethtool.

Signed-off-by: Sanjay Hortikar <horti@google.com>
Signed-off-by: Mahesh Bandewar <maheshb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>