net/devlink: Support querying hardware address of port function

PCI PF and VF devlink port can manage the function represented by
a devlink port.

Enable users to query port function's hardware address.

Example of a PCI VF port which supports a port function:
$ devlink port show pci/0000:06:00.0/2
pci/0000:06:00.0/2: type eth netdev enp6s0pf0vf1 flavour pcivf pfnum 0 vfnum 1
  function:
    hw_addr 00:11:22:33:44:66

$ devlink port show pci/0000:06:00.0/2 -jp
{
    "port": {
        "pci/0000:06:00.0/2": {
            "type": "eth",
            "netdev": "enp6s0pf0vf1",
            "flavour": "pcivf",
            "pfnum": 0,
            "vfnum": 1,
            "function": {
                "hw_addr": "00:11:22:33:44:66"
            }
        }
    }
}

Signed-off-by: Parav Pandit <parav@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>