iio: buffer: fix attach/detach pollfunc order

The original patch was error-ed by the submitter (me) and not by the author
(Lars).
After looking through the discussion logs (on email), it seems that this
order was wrong for the start, even though the order implemented in the
drivers was correct.

Discussions:
- first RFC: https://lore.kernel.org/linux-iio/20180622135322.3459-1-alexandru.ardelean@analog.com/
- 2nd patch: https://lore.kernel.org/linux-iio/20181219140912.22582-1-alexandru.ardelean@analog.com/
- final patch-sets:
  https://lore.kernel.org/linux-iio/20200522104632.517470-1-alexandru.ardelean@analog.com/
  https://lore.kernel.org/linux-iio/20200525113855.178821-1-alexandru.ardelean@analog.com/



The last one was applied.

The idea is that pollfunc should be attached before calling the
'indio_dev->setup_ops->postenable' hook and should be detached after
calling the 'indio_dev->setup_ops->predisable' hook.

While the drivers were updated to take this into account, the change to the
IIO core was somehow omitted and was made wrong.

This change fixes the order to the proper form.

Fixes f11d59d8: ("iio: Move attach/detach of the poll func to the core")
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>