iio:accel: preenable/postenable/predisable fixup for ST accel buffer

This patch is trying to cleanup for good the buffers operation functions.
There is no need of using preenable, all can be done into
postenable.
Let's also rename the goto label using operation to perform and not
where it fails.

Note, not stable material. This is cleaning up the logic flow rather
than fixing a bug.

Signed-off-by: Denis Ciocca <denis.ciocca@st.com>
Reviewed-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>