AP_MotorsMatrix: init to virtual and remove duplicate set initalised

libraries/AP_Motors/AP_MotorsMatrix.cpp

@@ -89,8 +89,6 @@ bool AP_MotorsMatrix::init(uint8_t expected_num_motors)
 
     set_update_rate(_speed_hz);
 
-    set_initialised_ok(true);
-
     return true;
 }
 #endif // ENABLE_SCRIPTING

libraries/AP_Motors/AP_MotorsMatrix.h

@@ -34,7 +34,7 @@ public:
 
 #ifdef ENABLE_SCRIPTING
     // Init to be called from scripting
-    bool                init(uint8_t expected_num_motors);
+    virtual bool        init(uint8_t expected_num_motors);
 #endif // ENABLE_SCRIPTING
 
     // set frame class (i.e. quad, hexa, heli) and type (i.e. x, plus)