Revert "dmaengine: virt-dma: don't always free descriptor upon completion"

This reverts commit b9855f03

.
The patch break existing DMA usage case. For example, audio SOC
dmaengine never release channel and cause virt-dma to cache too
much memory in descriptor to exhaust system memory.

Signed-off-by: Vinod Koul <vinod.koul@intel.com>