gnrc_uhcp: use actually advertised prefix length
gnrc_uhcp: use actually advertised prefix length

This makes the UHCP client to use the actually advertised prefix length
instead of assuming /64.

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