net: dsa: use ports list to setup default CPU port

Use the new ports list instead of iterating over switches and their
ports when setting up the default CPU port. Unassign it on teardown.

Now that we can iterate over multiple CPU ports, remove dst->cpu_dp.

At the same time, provide a better error message for CPU-less tree.

Signed-off-by: Vivien Didelot <vivien.didelot@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>