natmap: fix service not stopped when disabled in luci

luci removes `enable` option when disabled,
fix by setting default value to 0.

Signed-off-by: Richard Yu <yurichard3839@gmail.com>

net/natmap/files/natmap.init

@@ -17,7 +17,7 @@ load_interfaces() {
 
 validate_section_natmap() {
 	uci_load_validate "${NAME}" natmap "$1" "$2" \
-		'enable:bool:1' \
+		'enable:bool:0' \
 		'family:string' \
 		'udp_mode:bool:0' \
 		'interface:string' \