netfilter: nf_tables: skip transaction if update object is not implemented

Turn update into noop as a follow up for:

  9fedd894 ("netfilter: nf_tables: fix unexpected EOPNOTSUPP error")

instead of adding a transaction object which is simply discarded at a
later stage of the commit protocol.

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