dmaengine: sprd: Fix the incorrect start for 2-stage destination channels

The 2-stage destination channel will be triggered by source channel
automatically, which means we should not trigger it by software request.

Signed-off-by: Eric Long <eric.long@unisoc.com>
Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Vinod Koul <vkoul@kernel.org>