IMUAccelerometer: consistency of BoundCount checks

Similar to the guitar, only control[0] was checked, and that felt random.

Source/Core/InputCommon/ControllerEmu/ControlGroup/IMUAccelerometer.h

@@ -20,5 +20,7 @@ public:
   IMUAccelerometer(std::string name, std::string ui_name);
 
   std::optional<StateData> GetState() const;
+
+  bool AreInputsBound() const;
 };
 }  // namespace ControllerEmu