usb: f_fs: replace BUG in dead-code with less serious WARN_ON

Even though the BUG() in __ffs_event_add is a dead-code, it is still
better to warn rather then crash the system if that code ever gets
executed.
Reported-by: NFelipe Balbi <balbi@ti.com>
Signed-off-by: NMichal Nazarewicz <mina86@mina86.com>
Signed-off-by: NFelipe Balbi <balbi@ti.com>

drivers/usb/gadget/function/f_fs.c

@@ -2337,7 +2337,8 @@ static void __ffs_event_add(struct ffs_data *ffs,
 		break;
 
 	default:
-		BUG();
+		WARN(1, "%d: unknown event, this should not happen\n", type);
+		return;
 	}
 
 	{