dmaengine: imx-dma: convert callback to helper function

This is in preperation of moving to a callback that provides results to the
callback for the transaction. The conversion will maintain current behavior
and the driver must convert to new callback mechanism at a later time in
order to receive results.

Signed-off-by: Dave Jiang <[e-mail]>
Reviewed-by: Lars-Peter Clausen <[e-mail]>
Signed-off-by: Vinod Koul <[e-mail]>

drivers/dma/imx-dma.c

···
 out:
 	spin_unlock_irqrestore(&imxdma->lock, flags);
 
-	if (desc->desc.callback)
-		desc->desc.callback(desc->desc.callback_param);
-
+	dmaengine_desc_get_callback_invoke(&desc->desc, NULL);
 }
 
 static int imxdma_terminate_all(struct dma_chan *chan)