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 this, move mwifiex_usb_free() from mwifiex_usb_disconnect
to mwifiex_unregister_dev(). Function mwifiex_unregister_dev() is
called after flushing the command and RX work queues.
Suggested-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Ganapathi Bhat <gbhat@marvell.com>
Reviewed-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

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

@@ -644,8 +644,6 @@ static void mwifiex_usb_disconnect(struct usb_interface *intf)
 					 MWIFIEX_FUNC_SHUTDOWN);
 	}
 
-	mwifiex_usb_free(card);
-
 	mwifiex_dbg(adapter, FATAL,
 		    "%s: removing card\n", __func__);
 	mwifiex_remove_card(adapter);
@@ -1353,6 +1351,8 @@ static void mwifiex_unregister_dev(struct mwifiex_adapter *adapter)
 {
 	struct usb_card_rec *card = (struct usb_card_rec *)adapter->card;
 
+	mwifiex_usb_free(card);
+
 	mwifiex_usb_cleanup_tx_aggr(adapter);
 
 	card->adapter = NULL;