rtlwifi: rtl8192cu: Fix deadlock

Prevent code from calling itself indirectly, causing the driver to hang
and consume 100% CPU.

Without this fix, the following script can bring down a single CPU
system:
```
while true; do
  rmmod rtl8192cu
  modprobe rtl8192cu
done
```
Signed-off-by: Reto Schneider <code@reto-schneider.ch>
ACKed-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200622132113.14508-2-code@reto-schneider.ch

drivers/net/wireless/realtek/rtlwifi/usb.c

@@ -680,8 +680,10 @@ static void _rtl_usb_cleanup_rx(struct ieee80211_hw *hw)
 	tasklet_kill(&rtlusb->rx_work_tasklet);
 	cancel_work_sync(&rtlpriv->works.lps_change_work);
 
-	flush_workqueue(rtlpriv->works.rtl_wq);
-	destroy_workqueue(rtlpriv->works.rtl_wq);
+	if (rtlpriv->works.rtl_wq) {
+		destroy_workqueue(rtlpriv->works.rtl_wq);
+		rtlpriv->works.rtl_wq = NULL;
+	}
 
 	skb_queue_purge(&rtlusb->rx_queue);