Set default WAN config to Disable for all multi port devices

Changes to /etc/config/secn for the default WAN mode on TP devices with multiple Ethernet ports e.g. 741, 841, 842, 4300.
All devices are now configured to have the WAN interface disabled by default when first flashed. 
Previously multi port devices were configured to have WAN active as Ethernet but this leads to a conflict with the LAN Gateway setting.
Enabling WAN mode in operation will set up the LAN Gateway correctly.