backlight: sky81452-backlight: Fix refcount imbalance on error

When of_property_read_u32_array() returns an error code, a
pairing refcount decrement is needed to keep np's refcount
balanced.

Fixes: f705806c ("backlight: Add support Skyworks SKY81452 backlight driver")
Signed-off-by: Dinghao Liu <dinghao.liu@zju.edu.cn>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>