Overlay: Fix Hardkeys Value

* This is a sum not a single value (1 + 2 + 4 + 8 = 15)

Credits: @Plasmapump

Signed-off-by: Chet Kener <Cl3Kener@gmail.com>
Change-Id: I91e9707f608c99c4c9c76e281caa294ca44b9bc8