iio: dummy: iio_simple_dummy_buffer: use triggered buffer core calls

iio: dummy: iio_simple_dummy_buffer: use triggered buffer core calls

The iio_simple_dummy_configure_buffer() function is essentially a
re-implementation of the iio_triggered_buffer_setup() function.

This change makes use of the iio_triggered_buffer_setup() function. The
reason is so that we don't have to modify the iio_device_attach_buffer()
function in this driver as well.

One minor drawback is that the pollfunc name may not be 100% identical
with the one in the original code, but since it's an example, it should be
a big problem.

This change does a minor re-arranging of the included iio headers, as a
minor tidy-up.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20210215104043.91251-19-alexandru.ardelean@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>