gnrc_dhcpv6_client: dhcp-helper-function return valid seconds left
gnrc_dhcpv6_client: dhcp-helper-function return valid seconds left

dhcpv6_client_prefix_valid_until returned the valid timestamp
this fixes it according to the documentation given in
/sys/include/net/dhcpv6/client.h
to return seconds left

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