firewall3: use list notation for default zone network config to avoid "uci...

firewall3: use list notation for default zone network config to avoid "uci add_list" coercing the value wrongly

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@36806 3c298f89-4303-0410-b956-a3cf2f4a3e73