rtwn: use ieee80211_ht_check_tx_shortgi_20() and ieee80211_ht_check_tx_shortgi_40()

Use the new net80211 routines rather than rolling our own.

(The first version of this diff landed a previous version of what was
reviewed, so this brings it up to what was finally accepted in the
review.)

Differential Revision: https://reviews.freebsd.org/D47751

Reviewed by: bz