backlight: lm3630a: Initialize backlight_properties on init

The backlight_properties struct should be initialized to zero before
using, otherwise there will be some random values in the struct.

Fixes: 0c2a665a

 ("backlight: add Backlight driver for lm3630 chip")
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-v1-1-9ca62f7e4a33@z3ntu.xyz
Signed-off-by: Lee Jones <lee@kernel.org>