usb1: Fix exception raised during handleEvents.

Fixes errors like:
    File "usb1/__init__.py", line 2375, in handleEvents
      self.__mayRaiseUSBError(
    File "usb1/__init__.py", line 127, in mayRaiseUSBError
      if value < 0:
  TypeError: '<' not supported between instances of 'USBContext' and 'int'