minstrel: improve performance for non-MRR drivers

This patch enhances minstrel's performance for non-MRR setups,
by preventing it from sampling slower rates with >95% success
probability and by putting at least 1 non-sample frame between
several sample frames.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
diff --git a/net/mac80211/rc80211_minstrel.h b/net/mac80211/rc80211_minstrel.h
index 9a90a6a..869fe0e 100644
--- a/net/mac80211/rc80211_minstrel.h
+++ b/net/mac80211/rc80211_minstrel.h
@@ -16,6 +16,7 @@
 	unsigned int perfect_tx_time;
 	unsigned int ack_time;
 
+	int sample_limit;
 	unsigned int retry_count;
 	unsigned int retry_count_cts;
 	unsigned int retry_count_rtscts;
@@ -57,6 +58,7 @@
 
 	int n_rates;
 	struct minstrel_rate *r;
+	bool prev_sample;
 
 	/* sampling table */
 	u8 *sample_table;