netfilter: nf_tables: fix endian mismatch in return type

harmless, but it avoids sparse warnings:

nf_tables_api.c:2813:16: warning: incorrect type in return expression (different base types)
nf_tables_api.c:2863:47: warning: incorrect type in argument 3 (different base types)
nf_tables_api.c:3524:47: warning: incorrect type in argument 3 (different base types)
nf_tables_api.c:3538:55: warning: incorrect type in argument 3 (different base types)

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>