The 'device index' is the order number assigned by RetroArch for the controller, from list of the controllers detected. If you have just one controller connected, it will always be 1.
You can change the device index associated with a player from the Controls menu in the RGUI, under 'Port # Controls'. Given that you only have 1, you'll not be able to change it from the interface, since no other controller is present in the list.
Its like Retroarch is saying "Your N64 USB controller is a PS3 controller whether you like it or not".
It's not RetroArch that's determining the name, is the operating system that gives RetroArch the list of controllers with their names and characteristics. If your USB controller is telling the OS my name is PS3 Controller, this is now I identify myself, then this is what RetroArch shows.