net80211: add ieee80211_node_get_txrate() to populate the "new" transmit struct

ieee80211_node_get_txrate() populates the passed-in ieee80211_node_txrate with
the current rate configuration.

This is a no-op - nothing is yet setting VHT rates.

Differential Revision: https://reviews.freebsd.org/D48605
Reviewed by: bz