luci-app-firewall: show port opening cbi section independently of lan zone definition

Current the append traffic rules for "port forwarding" and "port opening"
to the router are only shown if 'wan' and 'lan' zone are defined at
once.

For "port opening" to the router only need a 'wan' zone. Removing
'lan' zone dependency for 'port opening' reflect this behavior.

Signed-off-by: Florian Eckert <fe@dev.tdt.de>