Input: Improve Controller Interface devices threading

This specific issue was already addressed by https://github.com/dolphin-emu/dolphin/pull/11635
though I felt like there was something more we could do, and wasn't too happy with the
likelihood of devices update calls being skipped (due to `m_devices_population_mutex` being locked).

Source/Core/InputCommon/ControllerInterface/DInput/DInputJoystick.cpp

@@ -222,7 +222,7 @@ bool Joystick::IsValid() const
   return SUCCEEDED(m_device->Acquire());
 }
 
-void Joystick::UpdateInput()
+Core::DeviceRemoval Joystick::UpdateInput()
 {
   HRESULT hr = 0;
 
@@ -261,6 +261,8 @@ void Joystick::UpdateInput()
   // try reacquire if input lost
   if (DIERR_INPUTLOST == hr || DIERR_NOTACQUIRED == hr)
     m_device->Acquire();
+
+  return Core::DeviceRemoval::Keep;
 }
 
 // get name

Source/Core/InputCommon/ControllerInterface/DInput/DInputJoystick.h

@@ -57,7 +57,7 @@ private:
   };
 
 public:
-  void UpdateInput() override;
+  Core::DeviceRemoval UpdateInput() override;
 
   Joystick(const LPDIRECTINPUTDEVICE8 device);
   ~Joystick();

Source/Core/InputCommon/ControllerInterface/DInput/DInputKeyboardMouse.cpp

@@ -205,7 +205,7 @@ void KeyboardMouse::UpdateCursorInput()
   m_state_in.cursor.y = (ControlState(point.y) / win_height * 2 - 1) * window_scale.y;
 }
 
-void KeyboardMouse::UpdateInput()
+Core::DeviceRemoval KeyboardMouse::UpdateInput()
 {
   UpdateCursorInput();
 
@@ -254,6 +254,8 @@ void KeyboardMouse::UpdateInput()
     else
       INFO_LOG_FMT(CONTROLLERINTERFACE, "Keyboard device failed to re-acquire, we'll retry later");
   }
+
+  return Core::DeviceRemoval::Keep;
 }
 
 std::string KeyboardMouse::GetName() const

Source/Core/InputCommon/ControllerInterface/DInput/DInputKeyboardMouse.h

@@ -94,7 +94,7 @@ private:
   };
 
 public:
-  void UpdateInput() override;
+  Core::DeviceRemoval UpdateInput() override;
 
   KeyboardMouse(const LPDIRECTINPUTDEVICE8 kb_device, const LPDIRECTINPUTDEVICE8 mo_device);
   ~KeyboardMouse();