network: radv: reorder functions

network: radv: reorder functions

In other files, we usually (but not always) place functions in the following order:
- network_adjust_xxx(), which applies default or updates settings
  specified in .network files,
- link_xxx_enabled(), which checks if the functionality is enabled,
- xxx_new() and xxx_free(), allocator and deallocator for sections,
- functions which apply/update/remove configs
- validators of section,
- conf parsers.

This does not change each function, but just changes the order.