backlight: as3711_bl: Simplify update_status

Replaces the open-coded checks of the state, with the
backlight_get_brightness() helper. This increases readability of the
code and align the functionality across the drivers.

Furthermore drop the debug prints in update_status().
If we need debug printing then we can add it to the backlight core.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>