staging: wfx: replace 0 by NULL

Replace 0 by NULL as the return value of a pointer-returning function.
Issue detected by sparse tool.
Signed-off-by: Jules Irenge <jbi.octave@gmail.com>
Link: https://lore.kernel.org/r/20191105010352.222479-2-jbi.octave@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/wfx/queue.c

@@ -565,7 +565,7 @@ struct hif_msg *wfx_tx_queues_get(struct wfx_dev *wdev)
 		}
 
 		if (ret)
-			return 0;
+			return NULL;
 
 		queue_num = queue - wdev->tx_queue;