FIFO in and out indices should be volatile.

Later code revealed that the receiving core never notices (reads anew) the _in member variable when waiting for the FIFO to be empty (_in == _out).

firmware/common/fifo.hpp

@@ -201,8 +201,8 @@ private:
 	}
 
 	T _data[size()];
-	size_t _in;
+	volatile size_t _in;
-	size_t _out;
+	volatile size_t _out;
 };
 
 #endif/*__FIFO_H__*/