dmaengine: ptdma: fix concurrency issue with multiple dma transfer

The command should be submitted only if the engine is idle,
for this, the next available descriptor is checked and set the flag
to false in case the descriptor is non-empty.

Signed-off-by: Sanjay R Mehta <sanju.mehta@amd.com>
Link: https://lore.kernel.org/r/1643814880-3882-2-git-send-email-Sanju.Mehta@amd.com


Signed-off-by: Vinod Koul <vkoul@kernel.org>