ramips: disable D-Link DIR-645 by default

Disable the D-Link DIR-645 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/18161/23
Signed-off-by: Jo-Philipp Wich <jo@mein.io>

target/linux/ramips/image/rt3883.mk

@@ -38,6 +38,7 @@ define Device/dir-645
   SEAMA_SIGNATURE := wrgn39_dlob.hans_dir645
   DEVICE_TITLE := D-Link DIR-645
   DEVICE_PACKAGES := kmod-usb-core kmod-usb-ohci kmod-usb2 swconfig
+  DEFAULT := n
 endef
 TARGET_DEVICES += dir-645