dmaengine: ipu: fix doc warning in ipu_irq.c

Fix the following make W=1 warning and correct description:

  drivers/dma/ipu/ipu_irq.c:238: warning: expecting prototype for ipu_irq_map(). Prototype was for ipu_irq_unmap() instead

Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Link: https://lore.kernel.org/r/20210603072425.2973570-1-yangyingliang@huawei.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>