net80211: convert ni_txrate to a struct, with extra rate information
- create struct ieee80211_node_txrate, which represents both the existing legacy / HT rates, and makes space for VHT rates
- convert the ieee80211_node_* routines that manipulate the txrate field to use the new format
- return OFDM6 for now if a VHT rate is set but the driver calls ieee80211_node_get_txrate_dot11rate(). It SHOULD be the lowest available rate - which for 11b will be a CCK rate, for turbo/half/quarter will be different rates! - but this is just for development.
This should be a no-op for existing drivers and rate control, as
everything now uses the accessor functions instead of directly
accessing ni->ni_txrate.
Locally tested:
- RTL8821AU, STA mode (5GHz VHT/40)
Differential Revision: https://reviews.freebsd.org/D48604
Reviewed by: bz, thj