iio: Fix parameters in iio_triggered_buffer_setup

This patch renames the top half handler and the bottom half handler
of iio_triggered_buffer_setup() in accordance with their usage.
The bottom half has been renamed to reflect the fact that it is a
thread based call, compliant with iio_alloc_pollfunc().
The names of the parameters were swapped, thus creating confusion.

Signed-off-by: Cristina Opriceana <cristina.opriceana@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>