netfilter: nf_tables_offload: fix always true policy is unset check

New smatch warnings:
net/netfilter/nf_tables_offload.c:316 nft_flow_offload_chain() warn: always true condition '(policy != -1) => (0-255 != (-1))'

Reported-by: kbuild test robot <lkp@intel.com>
Fixes: c9626a2c ("netfilter: nf_tables: add hardware offload support")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>