dissector: print hex_ascii combination in unkown linktypes

Commit f5c10ae4e3 ("dissector: do not panic on unknown hatype") let
netsniff-ng not panic anymore on unkown ha types, but without explicitly
starting netsniff-ng in hex or ascii printing mode, no dissection
will happen.

This patch fixes this situation so that at least ascii_hex() will be
called in case the link type is not known to us.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>