netfilter: nf_tables: bogus EOPNOTSUPP on basechain update

Userspace never includes the NFT_BASE_CHAIN flag, this flag is inferred
from the NFTA_CHAIN_HOOK atribute. The chain update path does not allow
to update flags at this stage, the existing sanity check bogusly hits
EOPNOTSUPP in the basechain case if the offload flag is set on.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>