r8169: fix mac address change

Network core refuses to change mac address because flag
IFF_LIVE_ADDR_CHANGE isn't set. Set this missing flag.

Fixes: 1f7aa2bc ("r8169: simplify rtl_set_mac_address")
Reported-by: Corinna Vinschen <vinschen@redhat.com>
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Tested-by: Corinna Vinschen <vinschen@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>