dmaengine: provide helper for setting txstate

Simple conditional struct filler to cut out some duplicated code.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>
diff --git a/drivers/dma/ioat/dma.h b/drivers/dma/ioat/dma.h
index 2339967..26f48ef 100644
--- a/drivers/dma/ioat/dma.h
+++ b/drivers/dma/ioat/dma.h
@@ -158,11 +158,7 @@
 	last_used = c->cookie;
 	last_complete = chan->completed_cookie;
 
-	if (txstate) {
-		txstate->last = last_complete;
-		txstate->used = last_used;
-		txstate->residue = 0;
-	}
+	dma_set_tx_state(txstate, last_complete, last_used, 0);
 
 	return dma_async_is_complete(cookie, last_complete, last_used);
 }