Use dmaenginem_async_device_register to simplify the code:
	remove dma_async_device_unregister.
	remove label unregister_slave, unregister_memcpy
Signed-off-by: Huang Shijie <sjhuang@iluvatar.ai>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Vinod Koul <vkoul@kernel.org>