[net80211] add VHT node flag; parsed chanwidth.

The VHT operational element (VHTOPMODE) isn't a uint32_t - it's
the MCS sets, freq1/freq2 parameters and channel width.
So, store the channel width too in lieu of just storing the
IE struct.

This changes the VHT parameter layout in ieee80211_node but it
doesn't change ABI at all.