ramips: disable D-Link DIR-300 B5/B6/B7 by default

Disable the D-Link DIR-300 B5/B6/B7 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/18161/18
Signed-off-by: Jo-Philipp Wich <jo@mein.io>

target/linux/ramips/image/rt305x.mk

@@ -237,6 +237,7 @@ define Device/dir-300-b7
   BLOCKSIZE := 4k
   IMAGE_SIZE := $(ralink_default_fw_size_4M)
   DEVICE_TITLE := D-Link DIR-300 B7
+  DEFAULT := n
 endef
 TARGET_DEVICES += dir-300-b7