USB: serial: fix stringify operator in usb-serial-simple
usb-serial-simple uses an unknown stringify macro that make
all drivers being named "stringify(vendor)".
This can be a problem when two drivers have the same (wrong) name:
kernel: usbcore: registered new interface driver usb_serial_simple
kernel: usbserial: USB Serial support registered for stringify(vendor)
kernel Error: Driver 'stringify(vendor)' is already registered, aborting...
kernel: usbserial: problem -16 when registering driver stringify(vendor)
kernel: usbserial: USB Serial deregistering driver stringify(vendor)
kernel: usbcore: deregistering interface driver usb_serial_simple