Correct SPL uses of MTD

This converts 2 usages of this option to the non-SPL form, since there is
no SPL_MTD defined in Kconfig

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/board/st/common/stm32mp_dfu.c b/board/st/common/stm32mp_dfu.c
index 0096f71..1cf4a3d 100644
--- a/board/st/common/stm32mp_dfu.c
+++ b/board/st/common/stm32mp_dfu.c
@@ -123,7 +123,7 @@
 			board_get_alt_info_mmc(dev, buf);
 	}
 
-	if (CONFIG_IS_ENABLED(MTD)) {
+	if (IS_ENABLED(CONFIG_MTD)) {
 		/* probe all MTD devices */
 		mtd_probe_devices();