net80211/ifconfig: swap IEEE80211_FVHT_USEVHT160 and 80P80

ieee80211_setupcurchan() compares the flags in a greater than manner.
In this case VHT160 should be > VHT80P80 as it is preferable.
Swap the two flags and add a comment to note this.

Sponsored by: The FreeBSD Foundation
Reviewed by: adrian
Differential Revision: https://reviews.freebsd.org/D48976

(cherry picked from commit 8f2e5b6ef32304ce340b91057b1eb4b191dbeac1)