Periodically try to activate DSHOT telemetry if enabled but not working

If DSHOT telemetry is enabled but one or more ESC's are not supplying valid telemetry packets, then send the DSHOT command to enable telemetry once a second while disarmed until all ESC's are supplying telemetry.

Addresses the issue of the flight controller booting without the ESC's powered. In this case the initial command at boot to enable bidirectional telemetry will be missed by the ESC since they're not powered. If the battery is subsequently plugged in the ESC's will default to bidirectional telemetry disabled.

This change will detect that ESC's are not supplying telemetry and attempt to preiodically enable them.

src/main/flight/mixer.c

@@ -1037,3 +1037,15 @@ float mixerGetLoggingThrottle(void)
 {
     return loggingThrottle;
 }
+
+#ifdef USE_DSHOT_TELEMETRY
+bool isDshotTelemetryActive(void)
+{
+    for (uint8_t i = 0; i < motorCount; i++) {
+        if (!isDshotMotorTelemetryActive(i)) {
+            return false;
+        }
+    }
+    return true;
+}
+#endif