Fix some compiler warnings

Marlin/Marlin_main.cpp

@@ -9630,7 +9630,7 @@ void quickstop_stepper() {
 
         #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/ubl_G29.cpp

@@ -608,7 +608,7 @@
     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.");
@@ -650,7 +650,7 @@
         return;
       }
 
-      uint16_t a = settings.calc_num_meshes();
+      int16_t a = settings.calc_num_meshes();
 
       if (!a) {
         SERIAL_PROTOCOLLNPGM("?EEPROM storage not available.");
@@ -1327,7 +1327,7 @@
    * use cases for the users. So we can wait and see what to do with it.
    */
   void unified_bed_leveling::g29_compare_current_mesh_to_stored_mesh() {
-    uint16_t a = settings.calc_num_meshes();
+    int16_t a = settings.calc_num_meshes();
 
     if (!a) {
       SERIAL_PROTOCOLLNPGM("?EEPROM storage not available.");

Marlin/ultralcd.cpp

@@ -2279,7 +2279,7 @@ 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)) {