dnsmasq: honor quietdhcp option for DHCPv6

Do not spam the syslog with DHCPv6 lease info if quietdhcp option
is selected. This already works for DHCPv4, make it work in the same
way for DHCPv6.

Signed-off-by: Arjen de Korte <build+lede@de-korte.org>
[Originally written by Arjen de Korte on GitHub but had issues providing
a SoB in correct format.]
Signed-off-by: Kevin Darbyshire-Bryant <kevin@darbyshire-bryant.me.uk>