net/mlx4_en: Save previous MAC address of the port so we can replace it later

In preparation to having more than one unicast MAC per port, we need to keep track
of the previous MAC address in the flow of ndo_set_mac_address,
so that mlx4_en_replace_mac will know what to replace.

Signed-off-by: Yan Burman <yanb@mellanox.com>
Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>