mwifiex: handle race during mwifiex_usb_disconnect

Race condition is observed during rmmod of mwifiex_usb:

1. The rmmod thread will call mwifiex_usb_disconnect(), download
   SHUTDOWN command and do wait_event_interruptible_timeout(),
   waiting for response.

2. The main thread will handle the response and will do a
   wake_up_interruptible(), unblocking rmmod thread.

3. On getting unblocked, rmmod thread  will make rx_cmd.urb = NULL in
   mwifiex_usb_free().

4. The main thread will try to resubmit rx_cmd.urb in
   mwifiex_usb_submit_rx_urb(), which is NULL.

To fix, wait for main thread to complete before calling
mwifiex_usb_free().

Signed-off-by: Ganapathi Bhat <gbhat@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/marvell/mwifiex/usb.c

@@ -644,6 +644,9 @@ static void mwifiex_usb_disconnect(struct usb_interface *intf)
 					 MWIFIEX_FUNC_SHUTDOWN);
 	}
 
+	if (adapter->workqueue)
+		flush_workqueue(adapter->workqueue);
+
 	mwifiex_usb_free(card);
 
 	mwifiex_dbg(adapter, FATAL,