iio: proximity: sx9500: fix iio_triggered_buffer_{predisable,postenable} positions

The iio_triggered_buffer_predisable() should be called last, to detach the
poll func after the devices has been suspended.

This change re-organizes things a bit so that the postenable & predisable
are symmetrical. It also converts the preenable() to a postenable().

Not stable material as there is no known problem with the current
code, it's just not consistent with the form we would like all the
IIO drivers to adopt so as to allow subsystem wide changes.

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