netfilter: nft_compat: ebtables 'nat' table is normal chain type

Unlike ip(6)tables, the ebtables nat table has no special properties.
This bug causes 'ebtables -A' to fail when using a target such as
'snat' (ebt_snat target sets ".table = "nat"').  Targets that have
no table restrictions work fine.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>