net: cpsw_new: Fix potential unregister of netdev that has not been registered yet

In the Linux kernel, the following vulnerability has been resolved:

net: cpsw_new: Fix potential unregister of netdev that has not been registered yet

If an error occurs during register_netdev() for the first MAC in

cpsw_register_ports(), even though cpsw->slaves[0].ndev is set to NULL,

cpsw->slaves[1].ndev would remain unchanged. This could later cause

cpsw_unregister_ports() to attempt unregistering the second MAC.

To address this, add a check for ndev->reg_state before calling

unregister_netdev(). With this change, setting cpsw->slaves[i].ndev

to NULL becomes unnecessary and can be removed accordingly.