netfilter: nf_nat_proto_icmpv6:: fix wrong comparison in icmpv6_manip_pkt

In commit 58a317f1 (netfilter: ipv6: add IPv6 NAT support), icmpv6_manip_pkt
was added with an incorrect comparison of ICMP codes to types.  This causes
problems when using NAT rules with the --random option.  Correct the
comparison.

This closes netfilter bugzilla #851, reported by Alexander Neumann.

Signed-off-by: Phil Oester <kernel@linuxace.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>