xilinx_axidma: Do not set DMA .notify to NULL after notify

If a stream notify function is not ready, it may re-populate the notify call-
back to indicate it should be re-polled later. This break in this usage, as
immediately following the notify() call, .notify is set to NULL. reverse the
ordering of the notify call and NULL assignment accordingly.

[PC: Reworked commit message]
Signed-off-by: NWendy Liang <jliang@xilinx.com>
Signed-off-by: NPeter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: NEdgar E. Iglesias <edgar.iglesias@gmail.com>

hw/dma/xilinx_axidma.c

@@ -514,8 +514,9 @@ static void axidma_write(void *opaque, hwaddr addr,
             break;
     }
     if (sid == 1 && d->notify) {
-        d->notify(d->notify_opaque);
+        StreamCanPushNotifyFn notifytmp = d->notify;
         d->notify = NULL;
+        notifytmp(d->notify_opaque);
     }
     stream_update_irq(s);
 }