Cap neighbour cost to INFINITY

With large txcost and rxcost, the calculated cost to a neighbour can
exceed 65535, i.e. INFINITY.  This is not a real issue, since any cost
over 65535 is equally seen as infinity.

However, it can confuse other tools (e.g. babelweb) who wouldn't
expect a cost higher than infinity.

neighbour.c

@@ -320,6 +320,6 @@ neighbour_cost(struct neighbour *neigh)
         b = MAX(b, 256);
         /* 1/(alpha * beta), which is just plain ETX. */
         /* Since a and b are capped to 16 bits, overflow is impossible. */
-        return (a * b + 128) >> 8;
+        return MIN((a * b + 128) >> 8, INFINITY);
     }
 }