mac80211: improve Minstrel variable & function naming

This patch ensures a consistent usage of variable names for type
"minstrel_rate_stats" to be used as "mrs" and from type minstrel_rate
as "mr" across both Minstrel & Minstrel-HT. In addition some
variable and function names got changed to more meaningful ones.

Signed-off-by: Thomas Huehn <thomas@net.t-labs.tu-berlin.de>
Acked-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/rc80211_minstrel_ht.h

@@ -78,7 +78,7 @@ struct minstrel_ht_sta {
 	u16 max_prob_rate;
 
 	/* time of last status update */
-	unsigned long stats_update;
+	unsigned long last_stats_update;
 
 	/* overhead time in usec for each frame */
 	unsigned int overhead;