BugLink: https://bugs.launchpad.net/bugs/1791953

[ Upstream commit b817047a ]

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>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>