(dhcpv6) don't override values the user just put in.

src/www/services_dhcpv6.php

@@ -309,7 +309,7 @@ if ($_SERVER['REQUEST_METHOD'] === 'GET') {
             }
             // router advertisement data lives in the same spot, copy
             foreach (array_keys($config['dhcpdv6'][$if]) as $fieldname) {
-                if (substr($fieldname,0 ,2) == 'ra') {
+                if (substr($fieldname,0 ,2) == 'ra' && !in_array($fieldname, array_keys($dhcpdconf))) {
                     $dhcpdconf[$fieldname] = $config['dhcpdv6'][$if][$fieldname];
                 }
             }