fix-wan-interface: fix CWMP bind interface

A ubus call is made for:

ubus call network.status '{"interface": "wan"}'

Where interface is retrieved from: cwmp.cpe.default_wan_interface

However, currently "device" is being used as the interface to bind to but this is the raw device which only works if a L3 device hasn't been created over the top - i.e for PPP.

Therefore pull L3 device instead, i.e:

"l3_device": "pppoe-wan", # <-- this is the one to use
"device": "eth0.1",

this also works when the WAN type is DHCP:

"l3_device": "eth0.1",
"device": "eth0.1",
"proto": "dhcp",