Copter: fixed interlock check on helis

the motors check is always false when disarmed, so can't be used for
arming check

ArduCopter/AP_Arming.cpp

@@ -597,7 +597,7 @@ bool AP_Arming_Copter::arm_checks(bool display_failure, bool arming_from_gcs)
 
     // if we are using motor interlock switch and it's enabled, fail to arm
     // skip check in Throw mode which takes control of the motor interlock
-    if (copter.ap.using_interlock && copter.motors->get_interlock()) {
+    if (copter.ap.using_interlock && copter.ap.motor_interlock_switch) {
         gcs().send_text(MAV_SEVERITY_CRITICAL,"Arm: Motor Interlock Enabled");
         return false;
     }