Fixed pcan Unpack error

[FedericoSpada]

One of my colleagues encountered a "not enough values to unpack" error while scanning for connected dongles with PCAN drivers installed. I couldn't replicate the error on my computer after installing the latest PCAN drivers. Still, analysing the instruction that raised that error, it seems that PCANBasic.GetValue() returned a wrong Tuple for a specific error condition. That method must always return a Tuple with 2 elements, otherwise PcanBus._detect_available_configs() raises a "not enough values to unpack" error at line 718.