net: ethernet: ti: am65-cpsw-nuss: Fix null pointer dereference for ndev

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

net: ethernet: ti: am65-cpsw-nuss: Fix null pointer dereference for ndev

In the TX completion packet stage of TI SoCs with CPSW2G instance, which has single external ethernet port, ndev is accessed without being initialized if no TX packets have been processed. It results into null pointer dereference, causing kernel to crash. Fix this by having a check on the number of TX packets which have been processed.