netfilter: nf_tables: Pass nf_hook_ops to nft_unregister_flowtable_hook()

The function accesses only the hook's ops field, pass it directly. This
prepares for nft_hooks holding a list of nf_hook_ops in future.

While at it, make use of the function in
__nft_unregister_flowtable_net_hooks() as well.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>