bluetooth: services: Set RTT estimate to NAN if negative

With the new MLT 2.1 metal fix we've observed that changes in
internal delays have caused us to produce negative TOF in some cases.

It's not possible for users to make sense of such results.
To avoid confusion we should set these as bad.

Signed-off-by: Olivier Lesage <[email protected]>
(cherry picked from commit 4e15fed)

Author: olivier-le-sage

subsys/bluetooth/cs_de/cs_de.c

@@ -272,7 +272,9 @@ static void calculate_dist_rtt(cs_de_report_t *p_report)
 		float rtt_distance_m = tof_ns * (SPEED_OF_LIGHT_M_PER_S / 1e9f);
 
 		for (uint8_t ap = 0; ap < p_report->n_ap; ap++) {
-			p_report->distance_estimates[ap].rtt = rtt_distance_m;
+			if (rtt_distance_m >= 0.0f) {
+				p_report->distance_estimates[ap].rtt = rtt_distance_m;
+			}
 		}
 	}
 }