iwn: use ieee80211_ht_check_tx_shortgi_20() and ieee80211_ht_check_tx_shortgi_40()

Use the new net80211 routines to check the node width and the
local / node flags.

This should be "more" correct than the previous code.

Differential Revision: https://reviews.freebsd.org/D47750

Reviewed by: bz