mlxsw: spectrum: Protect driver from buggy firmware

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

mlxsw: spectrum: Protect driver from buggy firmware

When processing port up/down events generated by the device's firmware,

the driver protects itself from events reported for non-existent local

ports, but not the CPU port (local port 0), which exists, but lacks a

netdev.

This can result in a NULL pointer dereference when calling

netif_carrier_{on,off}().

Fix this by bailing early when processing an event reported for the CPU

port. Problem was only observed when running on top of a buggy emulator.