[PATCH] USB: fix usb-serial leaks, oopses on disconnect

This fix addresses two issues:
- Unattached port structures were not freed
- My initial fix for crash when eventd runs a work in a freed port
  did not go far enough

Signed-off-by: Pete Zaitcev <zaitcev@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>