Removes some minor GCC warnings

Marlin/Marlin_main.cpp

@@ -5325,7 +5325,7 @@ inline void gcode_M200() {
     if (volumetric_enabled) {
       filament_size[target_extruder] = code_value_linear_units();
       // make sure all extruders have some sane value for the filament size
-      for (int i = 0; i < COUNT(filament_size); i++)
+      for (uint8_t i = 0; i < COUNT(filament_size); i++)
         if (! filament_size[i]) filament_size[i] = DEFAULT_NOMINAL_FILAMENT_DIA;
     }
   }
@@ -8695,6 +8695,6 @@ float calculate_volumetric_multiplier(float diameter) {
 }
 
 void calculate_volumetric_multipliers() {
-  for (int i = 0; i < COUNT(filament_size); i++)
+  for (uint8_t i = 0; i < COUNT(filament_size); i++)
     volumetric_multiplier[i] = calculate_volumetric_multiplier(filament_size[i]);
 }