dmaengine: consolidate assignment of DMA cookies

Everyone deals with assigning DMA cookies in the same way (it's part of
the API so they should be), so lets consolidate the common code into a
helper function to avoid this duplication.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Tested-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Jassi Brar <jassisinghbrar@gmail.com>
[imx-sdma.c & mxs-dma.c]
Tested-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>

drivers/dma/sirf-dma.c

@@ -257,13 +257,7 @@ static dma_cookie_t sirfsoc_dma_tx_submit(struct dma_async_tx_descriptor *txd)
 	/* Move descriptor to queue */
 	list_move_tail(&sdesc->node, &schan->queued);
 
-	/* Update cookie */
-	cookie = schan->chan.cookie + 1;
-	if (cookie <= 0)
-		cookie = 1;
-
-	schan->chan.cookie = cookie;
-	sdesc->desc.cookie = cookie;
+	cookie = dma_cookie_assign(txd);
 
 	spin_unlock_irqrestore(&schan->lock, flags);