Fix compiler warning: sketch/ultralcd.cpp:7258:61: warning: integer overflow in expression [-Woverflow]

  lcd_next_update_millis = millis() + (LCD_UPDATE_INTERVAL * 10000);

Use public interface instead of manipulating lcd_next_update_millis timer.

Firmware/ultralcd.cpp

@@ -6544,8 +6544,7 @@ bool lcd_selftest()
 	}
 	lcd_reset_alert_level();
 	enquecommand_P(PSTR("M84"));
-	lcd_clear();
-	lcd_next_update_millis = millis() + LCD_UPDATE_INTERVAL;
+	lcd_update_enable(true);
 	
 	if (_result)
 	{
@@ -7255,7 +7254,7 @@ static bool lcd_selftest_fan_dialog(int _fan)
 static int lcd_selftest_screen(int _step, int _progress, int _progress_scale, bool _clear, int _delay)
 {
 
-	lcd_next_update_millis = millis() + (LCD_UPDATE_INTERVAL * 10000);
+    lcd_update_enable(false);
 
 	int _step_block = 0;
 	const char *_indicator = (_progress > _progress_scale) ? "-" : "|";