iio:pressure: preenable/postenable/predisable fixup for ST press 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 use logical sequence of operations as
already done in accel driver.
Finally also rename the goto label using operation to perform and not
where it fails.

Not stable material as not fixing a 'bug' but rather bringing the
driver in line with general 'patterns' to allow a subsystem wide
cleanup.

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>