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.

This change reworks the predisable/postenable hooks so that the pollfunc is
attached/detached in the correct position.
It also balances the calls a bit, by grouping the preenable and the
iio_triggered_buffer_postenable() into a single
isl29125_buffer_postenable() function.
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>