net: vrf: Fix operation not supported when set vrf mac

Vrf device is not able to change mac address now because lack of
ndo_set_mac_address. Complete this in case some apps need to do
this.

Reported-by: Hui Wang <wanghui104@huawei.com>
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>