Currently factory.bin image recipe of ASUS RP-AC51 is not specified
explicitly and is thus set to the leaked one from the device recipe
right above, i.e. ASUS PL-AC56. Fix it to avoid potential breakage.
Fixes: 416d4483e878 ("ath79: add support for ASUS RP-AC51")
Signed-off-by: Sungbo Eo <mans0n@gorani.run>