net80211: fix VHT160/80P80/80 chanwidth selection in the "40-" case
Depending on the base channel ni_vht_chan2 - ni_vht_chan1 can be
negative. Apply abs() as indicated in the comments right above
| CCFS1 - CCFS0 | = 8 or > 16 |
in order to fix the channel width selection.
Sponsored by: The FreeBSD Foundation
PR: 293645
Fixes: 4bf049bfeefd9
MFC after: 3 days
Reviewed by: adrian
Differential Revision: https://reviews.freebsd.org/D55717