netfilter: nf_conntrack_dccp: use %s format string for buffer

Some invocations of nf_log_packet() use arg buffer directly instead of
"%s" format string with follow-up buffer pointer. Currently, these two
usages are not really critical, but we should fix this up nevertheless
so that we don't run into trouble if that changes one day.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>