setPWMLimit is ambiguous

[randaz81]

We should decide to use either machine units or percentage (eventually splitting into two different methods).
Otherwise, the output of the method is not comparable with getRefDutyCycle of the iPWMControl interface.
PS1: I was not able to find a units conversion helper in yarp dev sources, too.
PS2: Robot configuration files: probably startupMaxPWM parameter need to be updated.