stmmac: fix changing mac address

Without the IFF_LIVE_ADDR_CHANGE flag being set, the network code
disallows changing the mac address while the interface is UP.

Consequences are, for instance, that the interface can't be used
in a failover bond.

Add the missing flag to net_device priv_flags.

Tested on Intel Elkhart Lake with default settings, as well as with
failover and alb mode bonds.

Signed-off-by: Corinna Vinschen <vinschen@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>