backlight: mp3309c: Fully initialize backlight_properties during probe

props is stack allocated and, although this driver initializes all the
fields that are not "owned" by the framework, we'd still like to ensure
it is zeroed to avoid problems from this driver if the fields change.

Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>
Link: https://lore.kernel.org/r/20240220153532.76613-5-daniel.thompson@linaro.org
Signed-off-by: Lee Jones <lee@kernel.org>