iio: core: remove padding from private information

There was a recent discussion about this code:
  https://lore.kernel.org/linux-iio/20200322165317.0b1f0674@archlinux/



This looks like a good time to removed this, since any issues about it
should pop-up under testing, because the iio_dev is having a bit of an
overhaul and stuff being moved to iio_dev_opaque.

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