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/mv_xor.c b/drivers/dma/mv_xor.c
index 79fb1de..4b8c1fc 100644
--- a/drivers/dma/mv_xor.c
+++ b/drivers/dma/mv_xor.c
@@ -826,11 +826,7 @@
 	last_used = chan->cookie;
 	last_complete = mv_chan->completed_cookie;
 	mv_chan->is_complete_cookie = cookie;
-	if (txstate) {
-		txstate->last = last_complete;
-		txstate->used = last_used;
-		txstate->residue = 0;
-	}
+	dma_set_tx_state(txstate, last_complete, last_used, 0);
 
 	ret = dma_async_is_complete(cookie, last_complete, last_used);
 	if (ret == DMA_SUCCESS) {
@@ -842,12 +838,7 @@
 	last_used = chan->cookie;
 	last_complete = mv_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);
 }