mlxsw: spectrum: Guard against invalid local ports

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

mlxsw: spectrum: Guard against invalid local ports

When processing events generated by the device's firmware, the driver
protects itself from events reported for non-existent local ports, but
not for the CPU port (local port 0), which exists, but does not have all
the fields as any local port.

This can result in a NULL pointer dereference when trying access
'struct mlxsw_sp_port' fields which are not initialized for CPU port.

Commit 63b08b1f6834 ("mlxsw: spectrum: Protect driver from buggy firmware")
already handled such issue by bailing early when processing a PUDE event
reported for the CPU port.

Generalize the approach by moving the check to a common function and
making use of it in all relevant places.