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.
I think it is a minimum value for netdev options **not** defined in this header, i.e. the maximum value for generic options is the minimum value for device specific options.