dmaengine: mediatek: do not issue a new desc if one is still current

[ Upstream commit 2537b40b

 ]

Avoid issuing a new desc if one is still being processed as this can
lead to some desc never being marked as completed.

Signed-off-by: Guillaume Ranquet <granquet@baylibre.com>

Link: https://lore.kernel.org/r/20210513192642.29446-3-granquet@baylibre.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>