netfilter: nf_tables: Use nla_put_be32() to dump immediate parameters

nft_dump_register() should only be used with registers, not with
immediates.

Fixes: cb1b69b0 ("netfilter: nf_tables: add hash expression")
Fixes: 91dbc6be("netfilter: nf_tables: add number generator expression")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>