ar71xx/image: fix imagesize of template tplink-8mlzma

The new image size is verified by a running tplink device and checked
against mktplinkfw source code.

Signed-off-by: Alexander Couzens <lynxis@fe80.eu>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@45488 3c298f89-4303-0410-b956-a3cf2f4a3e73