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.
Because it is the maximum value defined here. And I want to discourage that inheriting types of `netdev_opt_types_t` pop up. This is not the reasoning behind this value. "Elsewhere" refers currently only to netapi and basic_net (see #1448), but other places where netapi is implemented are also thinkable.