enic: Feature Add: Add loopback capability to enic devices

Hardware has the loopback capability to queue the packets transmitted from
a device to the receive queue of the same device. enic now supports the
loopback capability.

Signed-off-by: Scott Feldman <scofeldm@cisco.com>
Signed-off-by: Vasanthy Kolluri <vkolluri@cisco.com>
Signed-off-by: Roopa Prabhu <roprabhu@cisco.com>
Signed-off-by: David S. Miller <davem@davemloft.net>