Fix M110 without leading N

Store N in the right variable.
This is too rarely used.

"mea culpa, mea culpa, mea maxima culpa"

Marlin/Marlin_main.cpp

 /**
  * GCode line number handling. Hosts may opt to include line numbers when
  * sending commands to Marlin, and lines will be checked for sequentiality.
- * M110 S<int> sets the current line number.
+ * M110 N<int> sets the current line number.
  */
 static long gcode_N, gcode_LastN, Stopped_gcode_LastN = 0;
 
  * M110: Set Current Line Number
  */
 inline void gcode_M110() {
-  if (code_seen('N')) gcode_N = code_value_long();
+  if (code_seen('N')) gcode_LastN = code_value_long();
 }
 
 /**