netfilter: nf_tables: correct NFT_LOGLEVEL_MAX value

should be same as NFT_LOGLEVEL_AUDIT, so use -, not +.

Fixes: 7eced5ab ("netfilter: nf_tables: add NFT_LOGLEVEL_* enumeration and use it")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>