auxdisplay: ht16k33: Extract ht16k33_brightness_set()

Extract brightness handling into a helper function, so it can be called
from multiple places.

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