Copter: disable ESC calibration for brushed motors

ArduCopter/esc_calibration.cpp

@@ -18,6 +18,11 @@ enum ESCCalibrationModes {
 // check if we should enter esc calibration mode
 void Copter::esc_calibration_startup_check()
 {
+    if (motors->get_pwm_type() >= AP_Motors::PWM_TYPE_BRUSHED16kHz) {
+        // ESC cal not valid for brushed motors
+        return;
+    }
+
 #if FRAME_CONFIG != HELI_FRAME
     // delay up to 2 second for first radio input
     uint8_t i = 0;