usb: device_next: allow to use label as interface string descriptor

The intention was to use the "interface-name" string property in the
interface string descriptor, but using the label property is acceptable
again. Therefore, allow the use of the DT label property string in the
interface string descriptor.

Follow exactly the same approach as in the CDC ACM implementation
introduced in the commit b0791400
("usb: device_next: cdc_acm: allow setting the interface description").

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