ar71xx: disable TP-Link TL-WA850RE v2 by default

Disable the TP-Link TL-WA850RE v2 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/18161/9
Signed-off-by: Jo-Philipp Wich <jo@mein.io>

target/linux/ar71xx/image/tiny-tp-link.mk

@@ -249,6 +249,7 @@ define Device/tl-wa850re-v2
   TPLINK_HWREV := 0
   IMAGE_SIZE := 3648k
   MTDPARTS := spi0.0:128k(u-boot)ro,1344k(kernel),2304k(rootfs),256k(config)ro,64k(art)ro,3648k@0x20000(firmware)
+  DEFAULT := n
 endef
 TARGET_DEVICES += tl-wa850re-v2