spl: Allow serial to be disabled in any XPL phase

The current check looks only at SPL, but TPL or VPL might have a
different setting. Update the condition.

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

common/spl/spl.c

@@ -837,7 +837,7 @@ void board_init_r(gd_t *dummy1, ulong dummy2)
  */
 void preloader_console_init(void)
 {
-#ifdef CONFIG_SPL_SERIAL
+#if CONFIG_IS_ENABLED(SERIAL)
 	gd->baudrate = CONFIG_BAUDRATE;
 
 	serial_init();		/* serial communications setup */