net: introduce new priv_flag indicating iface capable of change mac when running

Introduce IFF_LIVE_ADDR_CHANGE priv_flag and use it to disable
netif_running() check in eth_mac_addr()

Signed-off-by: Jiri Pirko <jpirko@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>