dmaengine: virt-dma: store result on dma descriptor

This allows each virtual channel to store information about each transfer
that completed, i.e. which transfer succeeded (or which failed) and if
there was any residue data on each (completed) transfer.

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