dmaengine: dmatest: unmap data on a single code-path when xfer done

After the DMA transfer is done, we don't need to call the un-mapping code
in 3 places. One is enough.

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