netdev: Provide maximum value of netdev_opt_t for further definitions

This simple addition allows for related definitions (e.g. `netapi_conf_type_t`) to easily find out, what the maximum possible value of `netdev_opt_t` is, even if this changes in the future.
The types in netapi and basic_net are no inheriting types FYI, they just take values from `netdev_opt_types_t` for convinience sake.