iio: light apds9960: Add the missing dev.parent

Without this, the iio:deviceX is missing in the /sys/bus/i2c/devices/0-0039
Some userspace tools use this path to identify a specific instance of the
device.

Signed-off-by: Yong Li <sdliyong@gmail.com>
Reviewed-By: Matt Ranostay <mranostay@gmail.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>