netfilter: nf_tables: fix unexpected EOPNOTSUPP error

If the object type doesn't implement an update operation and the user tries to
update it will silently ignore the update operation.

Fixes: aa4095a1

 ("netfilter: nf_tables: fix possible null-pointer dereference in object update")
Signed-off-by: Fernando Fernandez Mancera <ffmancera@riseup.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>