Minor string storage optimization

Marlin/src/gcode/queue.cpp

@@ -254,8 +254,8 @@ void GCodeQueue::ok_to_send() {
       while (NUMERIC_SIGNED(*p))
         SERIAL_ECHO(*p++);
     }
-    SERIAL_ECHOPGM(" P"); SERIAL_ECHO(int(BLOCK_BUFFER_SIZE - planner.movesplanned() - 1));
-    SERIAL_ECHOPGM(" B"); SERIAL_ECHO(BUFSIZE - length);
+    SERIAL_ECHOPAIR_P(SP_P_STR, int(BLOCK_BUFFER_SIZE - planner.movesplanned() - 1));
+    SERIAL_ECHOPAIR(" B", BUFSIZE - length);
   #endif
   SERIAL_EOL();
 }