auxdisplay: Make use of enum for backlight on / off

To turn the backlight on or off use our new enum CHARLCD_ON /
CHARLCD_OFF.

Reviewed-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Lars Poeschel <poeschel@lemonage.de>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>