af-packet: fix problem introduced in recent commit

Logic of patch 98e4a14f6d59fe8928fd6e2af3d9c3e8b42d00bf was correct
but implementation is wrong because TP_STATUS_KERNEL is equal to
zero and thus can not be evaluated in a binary operation. This patch
updates the logic by doing two tests.

Reported-by: Alessandro Guido

Ticket: https://redmine.openinfosecfoundation.org/issues/1081

PR builds:
- PR build: https://buildbot.suricata-ids.org/builders/regit/builds/90
- PR pcaps: https://buildbot.suricata-ids.org/builders/regit-pcap/builds/29