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.
What @OlegHahm says, but granted I cringed a little bit when I wrote this, myself. Any suggestions for a better doc?