net: dsa: use ports list in dsa_to_port

Use the new ports list instead of accessing the dsa_switch array
of ports in the dsa_to_port helper.

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