usb_add_function — add a function to a configuration
| int usb_add_function ( | struct usb_configuration * config, | 
| struct usb_function * function ); | 
   After initialization, each configuration must have one or more
   functions added to it.  Adding a function involves calling its bind()
   method to allocate resources such as interface and string identifiers
   and endpoints.
   
   This function returns the value of the function's bind, which is
   zero for success else a negative errno value.