net: dsa: use ports list to find a port by node

Use the new ports list instead of iterating over switches and their
ports to find a port from a given node.

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>