CVE-2026-31736: net: ethernet: mtk_ppe: avoid NULL deref when gmac0 is disabled

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

net: ethernet: mtkppe: avoid NULL deref when gmac0 is disabled

If the gmac0 is disabled, the precheck for a valid ingress device will cause a NULL pointer deref and crash the system. This happens because eth->netdev[0] will be NULL but the code will directly try to access netdevops.

Instead of just checking for the first netdevice, it must be checked if any of the mtketh netdevices is matching the netdevops of the ingress device.