net80211: rename \4 in IEEE80211_VHTCAP_BITS for clarity

Bits 2 and 3 (\3 and \4 of the %b flag mask) are the 'Supported Channel
Width Set' indicating VHT160 (B2) or VHT160 and VHT80P80 (B3) support.
Though longer rename \4 from CHAN80P80 to CHAN160+80P80 to not confuse
the reader that VHT160 might not be supported.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Reviewed by: adrian
Differential Revision: https://reviews.freebsd.org/D48977