ebtables: fix over-eager -o checks on custom chains

ebtables: fix over-eager -o checks on custom chains

Arturo reports ebtables-nft reports an error when -o is
used in custom chains:

-A MYCHAIN -o someif
makes ebtables-nft exit with an error:
"Use -o only in OUTPUT, FORWARD and POSTROUTING chains."

Problem is that all the "-o" checks expect <= NF_BR_POST_ROUTING
to mean "builtin", so -1 mistakenly leads to the checks being active.

Reported-by: Arturo Borrero Gonzalez <arturo@netfilter.org>
Closes: https://bugzilla.netfilter.org/show_bug.cgi?id=1347
Signed-off-by: Florian Westphal <fw@strlen.de>