net80211: improve media information for VHT5GHZ

Improve ieee80211_media_setup(), media2mode(), and
ieee80211_rate2media() for VHT5GHZ at least.

Reviewed by: adrian, gnn
MFC after: 2 weeks
Sponsored by: Rubicon Communications, LLC (d/b/a "Netgate")
Differential Revision: https://reviews.freebsd.org/D26089