dmaengine: dmatest: use dmaengine_terminate_sync() instead

The `dmaengine_terminate_all()` is marked as deprecated, so update the test
with `dmaengine_terminate_sync()` which is the recommended alternative.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Vinod Koul <vkoul@kernel.org>