Correct SPL use of BOOTSTAGE_FDT

This converts 1 usage of this option to the non-SPL form, since there is
no SPL_BOOTSTAGE_FDT defined in Kconfig

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/mips/lib/bootm.c b/arch/mips/lib/bootm.c
index 5fda914..5687ff2 100644
--- a/arch/mips/lib/bootm.c
+++ b/arch/mips/lib/bootm.c
@@ -284,7 +284,7 @@
 	if (CONFIG_IS_ENABLED(MALTA))
 		linux_extra = gd->ram_size;
 
-#if CONFIG_IS_ENABLED(BOOTSTAGE_FDT)
+#if IS_ENABLED(CONFIG_BOOTSTAGE_FDT)
 	bootstage_fdt_add_report();
 #endif
 #if CONFIG_IS_ENABLED(BOOTSTAGE_REPORT)