spektrum-scanner: fix plot_rssi.py script
spektrum-scanner: fix plot_rssi.py script

Adapt python script to comply with python coding conventions, i.e.
fixing error reported by flake8.

kaspar030/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,miri64/RIOT,ant9000/RIOT,miri64/RIOT,ant9000/RIOT,kaspar030/RIOT,miri64/RIOT,ant9000/RIOT,kaspar030/RIOT,jasonatran/RIOT,kaspar030/RIOT,jasonatran/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,miri64/RIOT,kaspar030/RIOT,jasonatran/RIOT,miri64/RIOT,jasonatran/RIOT,ant9000/RIOT,ant9000/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT