The AT DMA handler then accessed an
already freed packet.

Reported by Johannes Berg.
http://bugzilla.kernel.org/show_bug.cgi?id=9617

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Tested-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Jarod Wilson <jwilson@redhat.com>