dmaengine: tegra20-apb-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]>
Cc: Laxman Dewangan <[e-mail]>
Signed-off-by: Vinod Koul <[e-mail]>

drivers/dma/tegra20-apb-dma.c

···
 static void tegra_dma_tasklet(unsigned long data)
 {
 	struct tegra_dma_channel *tdc = (struct tegra_dma_channel *)data;
-	dma_async_tx_callback callback = NULL;
-	void *callback_param = NULL;
+	struct dmaengine_desc_callback cb;
 	struct tegra_dma_desc *dma_desc;
 	unsigned long flags;
 	int cb_count;
···
 		dma_desc  = list_first_entry(&tdc->cb_desc,
 					typeof(*dma_desc), cb_node);
 		list_del(&dma_desc->cb_node);
-		callback = dma_desc->txd.callback;
-		callback_param = dma_desc->txd.callback_param;
+		dmaengine_desc_get_callback(&dma_desc->txd, &cb);
 		cb_count = dma_desc->cb_count;
 		dma_desc->cb_count = 0;
 		spin_unlock_irqrestore(&tdc->lock, flags);
-		while (cb_count-- && callback)
-			callback(callback_param);
+		while (cb_count--)
+			dmaengine_desc_callback_invoke(&cb, NULL);
 		spin_lock_irqsave(&tdc->lock, flags);
 	}
 	spin_unlock_irqrestore(&tdc->lock, flags);