backlight: ktz8866: Correct the check for of_property_read_u32

of_property_read_u32 returns 0 when success, so reverse the
return value to get the true value.

Fixes: f8449c8f ("backlight: ktz8866: Add support for Kinetic KTZ8866 backlight")
Signed-off-by: Jianhua Lu <lujianhua000@gmail.com>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Link: https://lore.kernel.org/r/20240129122829.16248-1-lujianhua000@gmail.com


Signed-off-by: Lee Jones <lee@kernel.org>