USB: adutux: fix use-after-free on disconnect

The driver was clearing its struct usb_device pointer, which it used as
an inverted disconnected flag, before deregistering the character device
and without serialising against racing release().

This could lead to a use-after-free if a racing release() callback
observes the cleared pointer and frees the driver data before
disconnect() is finished with it.

This could also lead to NULL-pointer dereferences in a racing open().

Fixes: f08812d5

 ("USB: FIx locks and urb->status in adutux (updated)")
Cc: stable <stable@vger.kernel.org>     # 2.6.24
Reported-by:  <syzbot+0243cb250a51eeefb8cc@syzkaller.appspotmail.com>
Tested-by:  <syzbot+0243cb250a51eeefb8cc@syzkaller.appspotmail.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20190925092913.8608-1-johan@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>