AP_RPM: Move timeout check into synthetic quality check.

libraries/AP_RPM/AP_RPM.cpp

@@ -135,10 +135,6 @@ bool AP_RPM::healthy(uint8_t instance) const
     if (instance >= num_instances) {
         return false;
     }
-    // assume we get readings at at least 1Hz
-    if (AP_HAL::millis() - state[instance].last_reading_ms > 1000) {
-        return false;
-    }
 
     // check that data quality is above minimum required
     if (state[instance].signal_quality < _quality_min[0]) {

libraries/AP_RPM/RPM_PX4_PWM.cpp

@@ -120,6 +120,11 @@ void AP_RPM_PX4_PWM::update(void)
     if (count != 0) {
         state.last_reading_ms = AP_HAL::millis();
     }
+
+    // assume we get readings at at least 1Hz, otherwise reset quality to zero
+    if (AP_HAL::millis() - state.last_reading_ms > 1000) {
+        state.signal_quality = 0;
+    }
 }
 
 #endif // CONFIG_HAL_BOARD