wan: use separate `device` section for L2 options

On TOS 6.0+ do not store L2 options in interface section, use separate device section instead.

See:

• https://openwrt.org/docs/guide-user/base-system/basic-networking#device_sections
• https://openwrt.org/releases/21.02/notes-21.02.0#new_network_configuration_syntax_and_boardjson_change

For example:

config interface 'wan'
    option device 'eth0'
    option macaddr 'AA:BB:CC:11:22:33'

should rather be:

config interface 'wan'
    option device 'eth0'

config device
    option name 'eth0'
    option macaddr 'AA:BB:CC:11:22:33'