Fix sign-compare warnings in systems/robotInterfaces.
Fix sign-compare warnings in systems/robotInterfaces.

billhoffman/drake,billhoffman/drake,billhoffman/drake,sheim/drake,sheim/drake,billhoffman/drake,sheim/drake,billhoffman/drake,billhoffman/drake,billhoffman/drake,sheim/drake,sheim/drake,billhoffman/drake,sheim/drake,sheim/drake,sheim/drake