net: ethernet: ti: am65-cpsw-ethtool: configured critical setting only when no running netdevs

Ensure that critical setting can only be configured when there are no
running netdevs - all ports are down.

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>