contrib/wpa: pass IFM_IEEE80211_VHT5G if vht_enabled on the channel

Set media mode correctly to IFM_IEEE80211_VHT5G if vht_enabled is set
on the channel. Otherwise we'll end up setting 11NA.
Not a problem on first sight given net80211 does the upgrade to VHT
for us. But we would not set iv_des_mode ("desired mode") on the vap.
Setting this will put a contraint on/help scanning for our desired
(VHT) channels to my understanding of the code.

Sponsored by: The FreeBSD Foundation
MFC after: 6 weeks
Reviewed by: cy, adrian
Differential Revision: https://reviews.freebsd.org/D35978