NFLOG: don't crash trying to filter at link layer

Before:

@ tcpdump -i nflog icmp
tcpdump: WARNING: SIOCGIFADDR: nflog: No such device
Aborted (core dumped)

After:

@ tcpdump -i nflog icmp
tcpdump: WARNING: SIOCGIFADDR: nflog: No such device
tcpdump: NFLOG link-layer type filtering not implemented