net80211: (v)ht: use macros at hand

Rather than duplicating the manual logic here and leaving a comment,
use the self-explanatory macros we already have.

No functional changes intended.

Sponsored by: The FreeBSD Foundation
Reviewed by: adrian
Differential Revision: https://reviews.freebsd.org/D48359

(cherry picked from commit 5fdc4824a5e2646a07c0638eca9f5c81b0b85fd5)