pi_controller: fix pylint complaints
pi_controller: fix pylint complaints

JaviMerino/trappy,bjackman/trappy,derkling/trappy,ARM-software/trappy,bjackman/trappy,ARM-software/trappy,bjackman/trappy,sinkap/trappy,JaviMerino/trappy,joelagnel/trappy,bjackman/trappy,derkling/trappy,sinkap/trappy,ARM-software/trappy,sinkap/trappy,ARM-software/trappy,joelagnel/trappy,joelagnel/trappy,JaviMerino/trappy,derkling/trappy,sinkap/trappy,joelagnel/trappy