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.
Wow -  wasn't even close with my guess... ;-)
What about `/** minimum number of options defined elsewhere */` (ideally "elsewhere" should be replaced by where actually I would have to expect these options.
I also still don't get why a minimum value is named `..MAX`. Why not something like `NETDEV_MINIMUM_EXTERNAL_OPTS` or something along these lines?