dmaengine: iop-adma: 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/iop-adma.c

···
 		/* call the callback (must not sleep or submit new
 		 * operations to this channel)
 		 */
-		if (tx->callback)
-			tx->callback(tx->callback_param);
+		dmaengine_desc_get_callback_invoke(tx, NULL);
 
 		dma_descriptor_unmap(tx);
 		if (desc->group_head)