When r42245 moved A4001N to DT based detection the image padding size
was accidentally increased from 4 to 8 MiB. This made images unflashable
on these devices.

Fixes #19836.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@45945 3c298f89-4303-0410-b956-a3cf2f4a3e73