gre: remove setting default firewall zone to wan

gre: remove setting default firewall zone to wan

There are two problems with this behaviour that the zone is set to wan
if no zone config option is defined in the interface section.

* The zone for the interface is "normally" specified in the firewall
config file. So if we have defined "no" zone for this interface zone
option is set now to "wan" additonaly if we add the interface in the firewall
config section to the "lan" zone, the interface is added to lan and wan at once.

iptables-save | grep <iface>

This is not what I expect.

* If I do not want to set a zone to this interface it is not possible.

Remove the default assigment to wan if no zone option is defined.
If some one need the option it stil possible to define this option.

Signed-off-by: Florian Eckert <fe@dev.tdt.de>
Signed-off-by: Hans Dedecker <dedeckeh@gmail.com> [PKG_RELEASE increase]