Don't let MiRA trigger event-based probing if the current measurement

equals the average measurement, i.e. if the standard deviation is zero.

Change comparisons of current measurement to the average measurement
from >= to > in the "channel becomes good" check, and from <= to < in
the "channel becomes bad" check.

The paper's equations are written with <= and >= and thus so was our
implementation. But checking for equality makes no sense in the context
of event-triggered probing: The intention is to react to changes in
channel quality, which occur for instance when a laptop moves around
or when RF noise comes and goes. When the current measurement and the
average measurement are equal, this means channel quality has not
changed at all and starting to probe for a new rate is not necessary.

We should probably even add a margin to avoid triggering probing
based on small fluctuations, but this can be done later.

ok tb@

sys/net80211/ieee80211_mira.c

@@ -1,4 +1,4 @@
-/*	$OpenBSD: ieee80211_mira.c,v 1.24 2020/03/29 08:14:05 stsp Exp $	*/
+/*	$OpenBSD: ieee80211_mira.c,v 1.25 2020/03/30 19:10:42 stsp Exp $	*/
 
 /*
  * Copyright (c) 2016 Stefan Sperling <stsp@openbsd.org>
@@ -1193,7 +1193,7 @@ ieee80211_mira_choose(struct ieee80211_mira_node *mn, struct ieee80211com *ic,
 	}
 
 	/* Check if event-based probing should be triggered. */
-	if (g->measured <= g->average - 2 * g->stddeviation) {
+	if (g->measured < g->average - 2 * g->stddeviation) {
 		/* Channel becomes bad. Probe downwards. */
 		DPRINTFN(2, ("channel becomes bad; probe downwards\n"));
 		DPRINTFN(3, ("measured: %s Mbit/s\n",
@@ -1214,7 +1214,7 @@ ieee80211_mira_choose(struct ieee80211_mira_node *mn, struct ieee80211com *ic,
 		    (1 << ieee80211_mira_next_lower_intra_rate(mn, ni));
 #endif
 		ieee80211_mira_cancel_timeouts(mn);
-	} else if (g->measured >= g->average + 2 * g->stddeviation) {
+	} else if (g->measured > g->average + 2 * g->stddeviation) {
 		/* Channel becomes good. */
 		DPRINTFN(2, ("channel becomes good; probe upwards\n"));
 		DPRINTFN(3, ("measured: %s Mbit/s\n",