[net80211] Initial VHT node upgrade/downgrade support and initial IE parsing.

This is the bulk of the magic to start enabling VHT channel negotiation.
It is absolutely, positively not yet even a complete VHT wave-1 implementation.

• parse IEs in scan, assoc req/resp, probe req/resp;
• break apart the channel upgrade from the HT IE parsing - do it after the VHT IEs are parsed;
• (dirty! sigh) add channel width decision making in ieee80211_ht.c htinfo_update_chw(). This is the main bit where negotiated channel promotion through IEs occur.
• Shoehorn in VHT node init ,teardown, rate control, etc calls like the HT versions;
• Do VHT channel adjustment where appropriate

Tested:

• monitor mode, ath10k port
• STA mode, ath10k port - VHT20, VHT40, VHT80 modes

TODO:

• IBSS;
• hostap;
• (ignore mesh, wds for now);
• finish 11n state engine - channel width change, opmode notifications, SMPS, etc;
• VHT basic rate negotiation and acceptance criteria when scanning, associating, etc;
• VHT control/management frame handling (group managment and operating mode being the two big ones);
• Verify TX/RX VHT rate negotiation is actually working correctly.

Whilst here, add some comments about seqno allocation and locking. To achieve
the full VHT rates I need to push seqno allocation into the drivers and
finally remove the IEEE80211_TX_LOCK() I added years ago to fix issues. :/