docs: iio: triggered-buffers: use new helpers in example

docs: iio: triggered-buffers: use new helpers in example

Update the "typical" triggered buffer example to use various new helpers
that have been added in the last year or so. This reflects current
expectations of how similar code should be written.

Also zero-initialize the buffer so we don't leak stack data. And fix a
missing semicolon while we're at it.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Stepan Ionichev <sozdayvek@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>