ath: fix 20MHz HT short-gi logic.

This was broken in an earlier commit

(ca389486a9599768e0ba69dca13c208020623083).

PR: kern/285096
Differential Revision: https://reviews.freebsd.org/D49196
Fixes: ca389486a9599768e0ba69dca13c208020623083
Reviewed by: bz