dmaengine: at_xdmac: fix missing spin_unlock

Lock taken when entering the function but unlock missing before it
returns.

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>

drivers/dma/at_xdmac.c

@@ -901,6 +901,7 @@ at_xdmac_tx_status(struct dma_chan *chan, dma_cookie_t cookie,
 	 */
 	if (!desc->active_xfer) {
 		dma_set_residue(txstate, desc->xfer_size);
+		spin_unlock_bh(&atchan->lock);
 		return ret;
 	}