mlxsw: spectrum: Protect driver from buggy firmware

Vulnerability Description

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.

References

• https://git.kernel.org/stable/c/90d0736876c50ecde1a3275636a06b9ddb1cace9
• https://git.kernel.org/stable/c/da4d70199e5d82da664a80077508d6c18f5e76df
• https://git.kernel.org/stable/c/63b08b1f6834bbb0b4f7783bf63b80c8c8e9a047