Sub: fix compiler warnings on has_disarm_function

ArduSub/AP_Arming_Sub.cpp

@@ -12,7 +12,7 @@ bool AP_Arming_Sub::rc_calibration_checks(bool display_failure)
     return rc_checks_copter_sub(display_failure, channels);
 }
 
-const bool AP_Arming_Sub::has_disarm_function () {
+bool AP_Arming_Sub::has_disarm_function() const {
     bool has_shift_function = false;
     // make sure the craft has a disarm button assigned before it is armed
     // check all the standard btn functions

ArduSub/AP_Arming_Sub.h

@@ -13,7 +13,7 @@ public:
 
     bool rc_calibration_checks(bool display_failure) override;
     bool pre_arm_checks(bool display_failure) override;
-    const bool has_disarm_function();
+    bool has_disarm_function() const;
 
     bool disarm() override;
     bool arm(AP_Arming::Method method, bool do_arming_checks=true) override;