usb: device_next: avoid false error logging in CDC ECM/NCM

The interface descriptor and its associated string descriptor are shared
between different speed configurations. Do not try to add a string
descriptor if it has already been added and the index is not zero.

Similar to commit ec851ba7
("usb: device_next: avoid false error logging in CDC ACM").

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>