pkg/openwsn/sctimer_rtt: account for counters < uint32
pkg/openwsn/sctimer_rtt: account for counters < uint32

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