Fix compile issue with G38

Fixes #7250

.travis.yml

   - restore_configs
   - opt_enable NUM_SERVOS Z_ENDSTOP_SERVO_NR Z_SERVO_ANGLES DEACTIVATE_SERVOS_AFTER_MOVE
   - opt_enable AUTO_BED_LEVELING_3POINT DEBUG_LEVELING_FEATURE EEPROM_SETTINGS EEPROM_CHITCHAT
-  - opt_enable_adv EXTENDED_CAPABILITIES_REPORT AUTO_REPORT_TEMPERATURES AUTOTEMP
+  - opt_enable_adv EXTENDED_CAPABILITIES_REPORT AUTO_REPORT_TEMPERATURES AUTOTEMP G38_PROBE_TARGET
   - build_marlin
   #
   # Test MESH_BED_LEVELING feature, with LCD

Marlin/Marlin_main.cpp

 
       #if ENABLED(G38_PROBE_TARGET)
         case 38: // G38.2 & G38.3
-          if (subcode == 2 || subcode == 3)
-            gcode_G38(subcode == 2);
+          if (parser.subcode == 2 || parser.subcode == 3)
+            gcode_G38(parser.subcode == 2);
           break;
       #endif
 

Marlin/gcode.cpp

 char GCodeParser::command_letter;
 int GCodeParser::codenum;
 #if USE_GCODE_SUBCODES
-  int GCodeParser::subcode;
+  uint8_t GCodeParser::subcode;
 #endif
 
 #if ENABLED(FASTER_GCODE_PARSER)

Marlin/gcode.h

   static char command_letter;             // G, M, or T
   static int codenum;                     // 123
   #if USE_GCODE_SUBCODES
-    static int subcode;                   // .1
+    static uint8_t subcode;               // .1
   #endif
 
   #if ENABLED(DEBUG_GCODE_PARSER)