iio: buffer-dma: Move exports into IIO_DMA_BUFFER namespace

Avoid unnecessary pollution of the global symbol namespace and move
global APIs into a specific namespace. Import that into the places that
use them

Signed-off-by: Nuno Sa <nuno.sa@analog.com>
Link: https://patch.msgid.link/20240726-dev-iio-dma-buf-mod-namespace-v1-1-8877cfe7f68e@analog.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>