staging: wfx: wfx_flush() did not ensure that frames are processed

wfx_flush() exited once all frames are retrieved from the device.
However, it did not ensure they were processed by driver before to
return. Therefore, some frame may be processed after the interface has
disappear.

Change the place we signal that the queue is empty to fix that.
Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20200401110405.80282-30-Jerome.Pouiller@silabs.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/wfx/bh.c

@@ -108,8 +108,6 @@ static int rx_helper(struct wfx_dev *wdev, size_t read_len, int *is_cnf)
 			release_count = 1;
 		WARN(wdev->hif.tx_buffers_used < release_count, "corrupted buffer counter");
 		wdev->hif.tx_buffers_used -= release_count;
-		if (!wdev->hif.tx_buffers_used)
-			wake_up(&wdev->hif.tx_buffers_empty);
 	}
 	_trace_hif_recv(hif, wdev->hif.tx_buffers_used);
 
@@ -123,6 +121,8 @@ static int rx_helper(struct wfx_dev *wdev, size_t read_len, int *is_cnf)
 	skb_put(skb, hif->len);
 	// wfx_handle_rx takes care on SKB livetime
 	wfx_handle_rx(wdev, skb);
+	if (!wdev->hif.tx_buffers_used)
+		wake_up(&wdev->hif.tx_buffers_empty);
 
 	return piggyback;