auxdisplay: Use an enum for charlcd backlight on/off ops

We use an enum for calling the functions in charlcd, that turn the
backlight on or off. This enum is generic and can be used for other
charlcd turn on / turn off operations as well.

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