iio: pressure: zpa2326: fix iio_triggered_buffer_postenable position

The iio_triggered_buffer_{predisable,postenable} functions attach/detach
the poll functions.

The iio_triggered_buffer_postenable() should be called before (to attach
the poll func) and then the

The iio_triggered_buffer_predisable() function is hooked directly without
anything, which is probably fine, as the postenable() version seems to also
do some reset/wake-up of the device.
This will mean it will be easier when removing it; i.e. it just gets
removed.

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