cc2538: fix RSSI offset
cc2538: fix RSSI offset

RIOT-OS/RIOT,kYc0o/RIOT,ant9000/RIOT,jasonatran/RIOT,OlegHahm/RIOT,jasonatran/RIOT,authmillenon/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,ant9000/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,ant9000/RIOT,miri64/RIOT,miri64/RIOT,kaspar030/RIOT,ant9000/RIOT,miri64/RIOT,OlegHahm/RIOT,ant9000/RIOT,kYc0o/RIOT,OlegHahm/RIOT,authmillenon/RIOT,kYc0o/RIOT,OTAkeys/RIOT,authmillenon/RIOT,miri64/RIOT,authmillenon/RIOT,miri64/RIOT,OTAkeys/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,authmillenon/RIOT,kaspar030/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,jasonatran/RIOT,kYc0o/RIOT,jasonatran/RIOT,kaspar030/RIOT,kaspar030/RIOT,jasonatran/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,kaspar030/RIOT