AP_Arming: Use control_in rather then in_trim_dz() to cope with override only vehicles

libraries/AP_Arming/AP_Arming.cpp

@@ -567,7 +567,7 @@ bool AP_Arming::rc_arm_checks(AP_Arming::Method method)
                 if (c == nullptr) {
                     continue;
                 }
-                if (!c->in_trim_dz()) {
+                if (c->get_control_in() != 0) {
                     if ((method != Method::RUDDER) || (c != rc().get_arming_channel())) { // ignore the yaw input channel if rudder arming
                         check_failed(ARMING_CHECK_RC, true, "%s (RC%d) is not neutral", names[i], channels[i]);
                         check_passed = false;