net: Avoid NULL function pointer dereference on cleanup

The pSeries machine and some other devices don't supply a cleanup
callback. Revert part of 1ceef9f27359cbe92ef124bf74de6f792e71f6fb that
started calling it unconditionally.

Cc: Jason Wang <jasowang@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
Message-id: 1360707366-9271-1-git-send-email-afaerber@suse.de
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
