net: dsa: use ports list to find first CPU port

Use the new ports list instead of iterating over switches and their
ports when looking up the first CPU port in the 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>