sandbox: serial: Don't sync video in SPL

SPL does not support an LCD display so there is no need to sync the video
when there is serial output.

Signed-off-by: Simon Glass <sjg@chromium.org>

drivers/serial/sandbox.c

@@ -115,7 +115,9 @@ static int sandbox_serial_pending(struct udevice *dev, bool input)
 		return 0;
 
 	os_usleep(100);
+#ifndef CONFIG_SPL_BUILD
 	video_sync_all();
+#endif
 	if (next_index == serial_buf_read)
 		return 1;	/* buffer full */