gnrc_uhcpc: remove old prefix before adding new
gnrc_uhcpc: remove old prefix before adding new

OlegHahm/RIOT,kaspar030/RIOT,yogo1212/RIOT,basilfx/RIOT,ant9000/RIOT,OlegHahm/RIOT,kYc0o/RIOT,basilfx/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,yogo1212/RIOT,kYc0o/RIOT,jasonatran/RIOT,miri64/RIOT,kaspar030/RIOT,authmillenon/RIOT,jasonatran/RIOT,miri64/RIOT,ant9000/RIOT,OTAkeys/RIOT,yogo1212/RIOT,miri64/RIOT,OTAkeys/RIOT,kYc0o/RIOT,OlegHahm/RIOT,smlng/RIOT,basilfx/RIOT,authmillenon/RIOT,miri64/RIOT,kaspar030/RIOT,kYc0o/RIOT,yogo1212/RIOT,RIOT-OS/RIOT,basilfx/RIOT,authmillenon/RIOT,authmillenon/RIOT,jasonatran/RIOT,OlegHahm/RIOT,jasonatran/RIOT,authmillenon/RIOT,basilfx/RIOT,authmillenon/RIOT,smlng/RIOT,smlng/RIOT,miri64/RIOT,kaspar030/RIOT,OlegHahm/RIOT,smlng/RIOT,OTAkeys/RIOT,smlng/RIOT,ant9000/RIOT,ant9000/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,yogo1212/RIOT,OTAkeys/RIOT,kaspar030/RIOT,yogo1212/RIOT,OTAkeys/RIOT,ant9000/RIOT,kYc0o/RIOT