[net80211] add FVHT flags for channel widths.

The 11n code uses these bits for both configuration /and/ controlling
the channel width on softmac chips - it uses it to find the widest
width for all VAPs (eg a HT20 vap and a HT40 vap) to know what to
configure the ic_curchan.

For fullmac devices it isn't /as/ important, as each virtual device
exposed by the firmware will likely have its own configuration and the
firmware figures out what to do to enable it.