[net80211] create a helper function to calculate the station facing VHT capabilities.

This is needed for two reasons:

• Drivers will need to know what the negotiated set of VHT capabilities and rates are in order to configure (and reconfigure for opmode/chanwidth changes) how to speak to a given peer; and
• Because some vendors are "special", we should be careful in what we announce to them during peer association.

This isn't the complete solution, as I still need to make sure that when
sending out probe requests before we know what we want, we don't limit
the capabilities being announced. This is important for IBSS/mesh work
later on as probe request/response exchanges are the first hint at what
a peer supports. I'll look at adding that to the API soon.