netfilter: ctnetlink: use EOPNOTSUPP instead of EINVAL if the conntrack has no helper

This patch changes the return value if the conntrack has no helper assigned.
Instead of EINVAL, which is reserved for malformed messages, it returns
EOPNOTSUPP.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Patrick McHardy <kaber@trash.net>