USB backend: Add support for printers that don't report a serial number #170
Description
@wawaclem filed an Apple CUPS bug concerning Dymo printers and the USB backend.
Basically, Dymo doesn't report the printer serial number (a required value in the IEEE 1284 standard) in the device ID, which prevents you from using more than one Dymo printer connected to a given system.
The solution is to read the USB serial number string when the device ID does not contain a serial number.