HID: add driver for the Logitech Flight System G940

Implements a new USB-HID for Force Feedback based on the normal
Logitech Force Feedback code and FF-Memless.

Currently only supports the FF_CONSTANT effect although the joystick
appears to support additional non-standard ones.

Signed-off-by: Gary Stein <LordCnidarian@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

drivers/hid/hid-lg.h

@@ -15,4 +15,10 @@ int lg2ff_init(struct hid_device *hdev);
 static inline int lg2ff_init(struct hid_device *hdev) { return -1; }
 #endif
 
+#ifdef CONFIG_LOGIG940_FF
+int lg3ff_init(struct hid_device *hdev);
+#else
+static inline int lg3ff_init(struct hid_device *hdev) { return -1; }
+#endif
+
 #endif