ramips: disable TP-Link TL-WR840N v5 by default

Disable the TP-Link TL-WR840N v5 image by default as the device has
insufficient flash space for release build images.

Ref: https://forum.openwrt.org/t/devices-too-big-to-save-overlay/18161/29
Signed-off-by: Jo-Philipp Wich <jo@mein.io>

target/linux/ramips/image/mt76x8.mk

@@ -243,6 +243,7 @@ define Device/tl-wr840n-v5
   KERNEL_INITRAMFS := $(KERNEL_DTB) | tplink-v2-header -e
   IMAGE/sysupgrade.bin := tplink-v2-image -s -e | append-metadata | \
 	check-size $$$$(IMAGE_SIZE)
+  DEFAULT := n
 endef
 TARGET_DEVICES += tl-wr840n-v5