net80211: migrate ieee80211_rate2media() to use ieee80211_node_txrate

This is one of the two obvious left-over dot11rate lookups that trigger
a debug print. (The other is get_sta_info()).

• Change ieee80211_rate2media() to use the passed in ieee80211_node_txrate and use the rate type field to see if it's legacy/ht or vht.
• .. and (I hope!) actually handle the VHT rate correctly.
• Change the callers of ieee80211_rate2media() to use the new static initialisers.

For the one spot in ieee80211_ht.c which uses the HT initialiser, it should
just be a straight up mechanical change that correctly sets the HT MCS
rate.

For the rest of them in ieee80211.c, they're doing table initialisation
for the media types / rates, and it's currently very focused on legacy
rates. So just leave that all as-is for now with a mechanical change.

Differential Revision: https://reviews.freebsd.org/D48615