Fix compiler warnings

Marlin/src/feature/bedlevel/ubl/ubl_G29.cpp

@@ -609,7 +609,7 @@
     if (parser.seen('L')) {     // Load Current Mesh Data
       g29_storage_slot = parser.has_value() ? parser.value_int() : storage_slot;
 
-      uint16_t a = settings.calc_num_meshes();
+      int16_t a = settings.calc_num_meshes();
 
       if (!a) {
         SERIAL_PROTOCOLLNPGM("?EEPROM storage not available.");
@@ -651,7 +651,7 @@
         return;
       }
 
-      uint16_t a = settings.calc_num_meshes();
+      int16_t a = settings.calc_num_meshes();
 
       if (!a) {
         SERIAL_PROTOCOLLNPGM("?EEPROM storage not available.");

Marlin/src/gcode/bedlevel/M420.cpp

@@ -54,7 +54,7 @@ void GcodeSuite::M420() {
 
       #if ENABLED(EEPROM_SETTINGS)
         const int8_t storage_slot = parser.has_value() ? parser.value_int() : ubl.storage_slot;
-        const uint16_t a = settings.calc_num_meshes();
+        const int16_t a = settings.calc_num_meshes();
 
         if (!a) {
           SERIAL_PROTOCOLLNPGM("?EEPROM storage not available.");

Marlin/src/lcd/ultralcd.cpp

@@ -2285,7 +2285,7 @@ void kill_screen(const char* lcd_msg) {
      *    Save Bed Mesh
      */
     void _lcd_ubl_storage_mesh() {
-      uint16_t a = settings.calc_num_meshes();
+      int16_t a = settings.calc_num_meshes();
       START_MENU();
       MENU_BACK(MSG_UBL_LEVEL_BED);
       if (!WITHIN(ubl_storage_slot, 0, a - 1)) {