netfilter: physdev: physdev-is-out should not work with OUTPUT chain

physdev_mt() will check skb->nf_bridge first, which was alloced in
br_nf_pre_routing. So if we want to use --physdev-out and physdev-is-out,
we need to match it in FORWARD or POSTROUTING chain. physdev_mt_check()
only checked physdev-out and missed physdev-is-out. Fix it and update the
debug message to make it clearer.

Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: Marcelo R Leitner <marcelo.leitner@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>