iio: dln2-adc: fix iio_triggered_buffer_postenable() position

The iio_triggered_buffer_postenable() hook should be called first to
attach the poll function. The iio_triggered_buffer_predisable() hook is
called last (as is it should).

This change moves iio_triggered_buffer_postenable() to be called first. It
adds iio_triggered_buffer_predisable() on the error paths of the postenable
hook.
For the predisable hook, some code-paths have been changed to make sure
that the iio_triggered_buffer_predisable() hook gets called in case there
is an error before it.

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