dmaengine: pl08x: drop the useless function

Unneeded variable: "retval". Return "NULL" , so we have to make code clear.
better way, drop the function.

Signed-off-by: Haowen Bai <baihaowen@meizu.com>
Link: https://lore.kernel.org/r/1649726180-13133-1-git-send-email-baihaowen@meizu.com


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