netfilter: nf_tables: Flowtable hook's pf value never varies

When checking for duplicate hooks in nft_register_flowtable_net_hooks(),
comparing ops.pf value is pointless as it is always NFPROTO_NETDEV with
flowtable hooks.

Dropping the check leaves the search identical to the one in
nft_hook_list_find() so call that function instead of open coding.

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