Serial buffer over run work around for v2.0.0 (#9236)

* Work around for serial buffer over run

PronterFace sends a lot of M105 commands.  During long operations such
as UBL's G29 P1, G29 P2, G29 P4 and G26 this can over run the serial
buffer.   This results (very often) in a M1 (actually a M1M105) ending
up in the command queue.

Until we figure out a better way to resolve this issue, this will keep
the UBL commands from experiencing bogus commands at thier completion.

Marlin/src/gcode/queue.cpp

@@ -90,8 +90,7 @@ void queue_setup() {
  * Clear the Marlin command queue
  */
 void clear_command_queue() {
-  cmd_queue_index_r = cmd_queue_index_w;
-  commands_in_queue = 0;
+  cmd_queue_index_r = cmd_queue_index_w = commands_in_queue = 0;
 }
 
 /**