backlight: lm3630a: Use backlight_get_brightness helper in update_status

As per documentation "drivers are expected to use this function in their
update_status() operation to get the brightness value.".

With this we can also drop the manual backlight_is_blank() handling
since backlight_get_brightness() is already handling this correctly.

Signed-off-by: Luca Weiss <luca@z3ntu.xyz>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Link: https://lore.kernel.org/r/20240220-lm3630a-fixups-v1-3-9ca62f7e4a33@z3ntu.xyz
Signed-off-by: Lee Jones <lee@kernel.org>