netfilter: nf_tables: allow to register flowtable with no devices

A flowtable might be composed of dynamic interfaces only. Such dynamic
interfaces might show up at a later stage. This patch allows users to
register a flowtable with no devices. Once the dynamic interface becomes
available, the user adds the dynamic devices to the flowtable.

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