Add a WifiDataClass struct, that is immutable and contains all the
function pointers that were previously embedded in WifiData directly.
They are not ever modified after creation, hence this allows to have
a "static const" allocated instance of the VTable.

Also rename wifi_data_deinit() to wifi_data_unref(). It does not only
deinitialize the instance, instead it also frees it. Hence, rename it
to "unref()".