net80211: add a driver-private pointer to struct ieee80211_node

Add a void *ni_drv_data field to struct ieee80211_node that drivers
can use to backtrack to their internal state from a net80211 node.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
X-Differential Revision: https://reviews.freebsd.org/D30654 (abandoned)