iio: potentiostat: lmp9100: fix iio_triggered_buffer_{predisable,postenable} positions

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

For the predisable hook, the disable code should occur before detaching
the poll func, and for the postenable hook, the poll func should be
attached before the enable code.

The lmp9100 was attaching a poll function but never detaching it via any
IIO disable hook.

This change adds the detaching of the poll function, and moves/renames
lmp91000_buffer_preenable() function to lmp91000_buffer_postenable().
The idea is to make it more symmetrical, so that when the
iio_triggered_buffer_{predisable,postenable} functions get removed, it's
easier to see.

Fixes: 67e17300 ("iio: potentiostat: add LMP91000 support")
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>