ar71xx: set US region code for TP-Link TL-WR710N v1 image

Non-US versions of the TP-Link TL-WR710N v1 don't have a region code so
far, so we can just set US unconditionally.

Signed-off-by: Matthias Schiffer <[email protected]>

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

@@ -747,6 +747,7 @@ define Device/tl-wr710n-v1
   DEVICE_PROFILE := TLWR710
   TPLINK_HWID := 0x07100001
   CONSOLE := ttyATH0,115200
+  IMAGE/factory.bin := append-rootfs | mktplinkfw factory -C US
 endef
 
 define Device/tl-wr710n-v2