iio: light: lm3533-als: use iio_device_set_parent() to assign parent

This assignment is the more peculiar as it assigns the parent of the
platform-device's device (i.e. pdev->dev.parent) as the IIO device's
parent.

Since the devm_iio_device_alloc() [now] assigns the device argument as the
default parent (and since this is the more common case), for cases
where the parent needs to be different, the iio_device_set_parent helper
should be used.

That makes things a bit more obvious about the new behavior of
devm_iio_device_alloc() and makes it clearer that iio_device_set_parent()
should be used.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>