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