netfilter: nf_tables: skip netdev hook unregistration if table is dormant

Skip hook unregistration when adding or deleting devices from an
existing netdev basechain. Otherwise, commit/abort path try to
unregister hooks which not enabled.

Fixes: b9703ed4 ("netfilter: nf_tables: support for adding new devices to an existing netdev chain")
Fixes: 7d937b10 ("netfilter: nf_tables: support for deleting devices in an existing netdev chain")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>