enic: Add new fw devcmd to set mac address of an interface

This patch adds a new devcmd CMD_SET_MAC_ADDR to set the mac address of an
interface.

Signed-off-by: Roopa Prabhu <roprabhu@cisco.com>
Signed-off-by: Christian Benvenuti <benve@cisco.com>
Signed-off-by: Sujith Sankar <ssujith@cisco.com>
Signed-off-by: David S. Miller <davem@davemloft.net>