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/shdma.c b/drivers/dma/shdma.c
index 8aeda9c..98f82cd 100644
--- a/drivers/dma/shdma.c
+++ b/drivers/dma/shdma.c
@@ -752,12 +752,7 @@
 	last_used = chan->cookie;
 	last_complete = sh_chan->completed_cookie;
 	BUG_ON(last_complete < 0);
-
-	if (txstate) {
-		txstate->last = last_complete;
-		txstate->used = last_used;
-		txstate->residue = 0;
-	}
+	dma_set_tx_state(txstate, last_complete, last_used, 0);
 
 	spin_lock_bh(&sh_chan->desc_lock);