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)

sys/net80211/ieee80211_node.h

@@ -260,6 +260,8 @@ struct ieee80211_node {
 	/* U-APSD */
 	uint8_t			ni_uapsd;	/* U-APSD per-node flags matching WMM STA QoS Info field */
 
+	void			*ni_drv_data;	/* driver specific */
+
 	uint64_t		ni_spare[3];
 };
 MALLOC_DECLARE(M_80211_NODE);