net80211: change order in ieee80211_vhtchanflags()

While 80P80 is less likely to be used, VHT160 (a single contiguous
width) is harder to acquire but also preferable so return that first.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: adrian
Differential Revision: https://reviews.freebsd.org/D35977