dt-bindings: auxdisplay: ht16k33: Fix default-brightness-level range

V4 changed the range from 0..15 to 1..16 in the driver, to match the
dimming set hardware register, but forgot to update the DT binding
documentation.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Robin van der Gracht <robin@protonic.nl>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>