Correct SPL use of DM_RNG

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

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/boot/vbe_request.c b/boot/vbe_request.c
index 45f1d2b..312edfa 100644
--- a/boot/vbe_request.c
+++ b/boot/vbe_request.c
@@ -36,7 +36,7 @@
 	u32 size;
 	int ret;
 
-	if (!CONFIG_IS_ENABLED(DM_RNG))
+	if (!IS_ENABLED(CONFIG_DM_RNG))
 		return -ENOTSUPP;
 
 	if (ofnode_read_u32(node, "vbe,size", &size)) {