Rover: set failsafe_radio flag when RC failsafe is triggered/cleared

This sets failsafe_radio flag when RC failsafe is triggered/cleared which is further used in AP_Arming::manual_transmitter_checks to fail pre-arm checks if failsafe is on

Rover/radio.cpp

@@ -152,5 +152,6 @@ void Rover::radio_failsafe_check(uint16_t pwm)
     if (AP_HAL::millis() - failsafe.last_valid_rc_ms > 500) {
         failed = true;
     }
+    AP_Notify::flags.failsafe_radio = failed;
     failsafe_trigger(FAILSAFE_EVENT_THROTTLE, "Radio", failed);
 }