netfilter: bridge: fix module autoload in compat case

We expected 0 if module doesn't exist, which is no longer the case
(42046e2e

,
netfilter: x_tables: return -ENOENT for non-existant matches/targets).

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