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

drivers/dma/ipu/ipu_irq.c

@@ -230,7 +230,7 @@ int ipu_irq_map(unsigned int source)
 }
 
 /**
- * ipu_irq_map() - map an IPU interrupt source to an IRQ number
+ * ipu_irq_unmap() - unmap an IPU interrupt source
  * @source:	interrupt source bit position (see ipu_irq_map())
  * @return:	0 or negative error code
  */