gluon-core: don't request a prefix via DHCPv6 on WAN

The prefix is not used, and requesting it leads to odhcp6c log spam with
certain DHCPv6 servers.

package/gluon-core/luasrc/lib/gluon/upgrade/110-network

@@ -24,6 +24,7 @@ uci:section('network', 'interface', 'wan6', {
 	peerdns = false,
 	ip6table = 1,
 	sourcefilter = false,
+	reqprefix = 'no',
 })
 
 if not uci:get('network', 'wan6', 'proto') then