net: ethernet: ti: am65-cpsw: fix error handling in am65_cpsw_nuss_probe()

The am65_cpsw_nuss_cleanup_ndev() function calls unregister_netdev()
even if register_netdev() fails, which triggers WARN_ON(1) in
unregister_netdevice_many(). To fix it, make sure that
unregister_netdev() is called only on registered netdev.

Compile tested only.

Fixes: 84b4aa49

 ("net: ethernet: ti: am65-cpsw: add multi port support in mac-only mode")
Signed-off-by: Zhang Changzhong <zhangchangzhong@huawei.com>
Reviewed-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>