[net80211] add methods to fetch the global and per-VAP WME parameters.

For now there isn't any per-VAP WME state. The eventual aim is to migrate
the driver direct use of WME parameters over to use these methods as
appropriate (global for most devices, per-VAP for firmware NICs that support
it) in preparation for actual per-VAP WME (and other thing) state change
support.