iio: hdc100x: fix iio_triggered_buffer_{predisable,postenable} positions

The iio_triggered_buffer_postenable() hook should be called first to
attach the poll function and the iio_triggered_buffer_predisable() hook
should be called last in the predisable hook.

This change updates the driver to attach/detach the poll func in the
correct order.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Reviewed-by: Matt Ranostay <matt.ranostay@konsulko.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>