ramips: mt7621: disable images for gehua_ghl-r-001

This device OOPs during the boot due to broken flash. It can be probably
fixed with `broken-flash-reset` once ramips is on 4.19 kernel.

So disable images for this device until its fixed.

Ref: FS#2695, PR#2483
Signed-off-by: Petr Štetiar <ynezz@true.cz>

target/linux/ramips/image/mt7621.mk

@@ -301,6 +301,7 @@ define Device/gehua_ghl-r-001
   DEVICE_MODEL := GHL-R-001
   DEVICE_PACKAGES := kmod-mt7603 kmod-mt76x2 kmod-usb3 \
 	kmod-usb-ledtrig-usbport wpad-basic
+  DEFAULT := n
 endef
 TARGET_DEVICES += gehua_ghl-r-001