Fix checksum + card.saving

Followup to #10204

Marlin/Marlin_main.cpp

@@ -1070,10 +1070,12 @@ inline void get_serial_commands() {
 
         gcode_LastN = gcode_N;
       }
-      else if (card.saving) {
+      #if ENABLED(SDSUPPORT)
-        gcode_line_error(PSTR(MSG_ERR_NO_CHECKSUM));
+        else if (card.saving) {
-        return;
+          gcode_line_error(PSTR(MSG_ERR_NO_CHECKSUM));
-      }
+          return;
+        }
+      #endif
 
       // Movement commands alert when stopped
       if (IsStopped()) {