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