ar71xx/image: change unifi UBNT_CHIP to ar7240

Verified by OEM firmware images which contains the same
chip.

Tested-by: Andreas Bräu <[email protected]>
Signed-off-by: Alexander Couzens <[email protected]>

target/linux/ar71xx/image/ubnt.mk

@@ -63,7 +63,7 @@ endef
 define Device/ubnt-bz
   $(Device/ubnt)
   UBNT_TYPE := BZ
-  UBNT_CHIP := ar934x
+  UBNT_CHIP := ar7240
 endef
 
 define Device/rw2458n