ar71xx: fix tl-wdr3320-v2 upgrade

Fix the error that tl-wdr3320-v2 can't upgrade firmware via web
interface by using magic_ver="0200" for this device.

Signed-off-by: 南浦月 <nanpuyue@gmail.com>
[commit message facelift]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
(cherry picked from commit 0ff2385a92)

target/linux/ar71xx/base-files/lib/upgrade/platform.sh

@@ -499,7 +499,7 @@ platform_check_image() {
 		local magic_ver="0100"
 
 		case "$board" in
-		tl-wdr6500-v2)
+		tl-wdr3320-v2|tl-wdr6500-v2)
 			magic_ver="0200"
 			;;
 		esac