mlxsw: spectrum_router: Generalize mlxsw_sp_netdevice_router_port_event()

Prepare mlxsw_sp_netdevice_router_port_event() for handling of
NETDEV_PRE_CHANGEADDR. Split out the part that deals with the actual
changes and call it for the two events currently handled.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>