net80211: correct IEEE80211_FEXT_BITS

Correct IEEE80211_FEXT_BITS adding the missed one and move the later
one further. This will actually make VHT to show.

Sponsored by: The FreeBSD Foundation
Reviewed by: adrian
Differential Revision: https://reviews.freebsd.org/D35977

(cherry picked from commit 463d6125e7bff6504099f34e4fe7aa0a68166e6e)