net: mvneta: enable change MAC address when interface is up

Function eth_prepare_mac_addr_change() is called as part of MAC
address change. This function check if interface is running.
To enable change MAC address when interface is running:
IFF_LIVE_ADDR_CHANGE flag must be set to dev->priv_flags field

Fixes: c5aff182 ("net: mvneta: driver for Marvell Armada 370/XP
network unit")
Cc: stable@vger.kernel.org
Signed-off-by: Dmitri Epshtein <dima@marvell.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>