acm uses a workqueue to defer part of a write operation.
In case of disconnect this work must be waited for.
  - fix race between write and disconnect
Signed-off-by: Oliver Neukum <oliver@neukum.name>
Signed-off-by: Vojtech Pavlik <vojtech@suse.cz>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>