Correct SPL use of SL28_SPL_LOADS_OPTEE_BL32

This converts 1 usage of this option to the non-SPL form, since there is
no SPL_SL28_SPL_LOADS_OPTEE_BL32 defined in Kconfig

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/board/kontron/sl28/sl28.c b/board/kontron/sl28/sl28.c
index 2579236..e78a0d5 100644
--- a/board/kontron/sl28/sl28.c
+++ b/board/kontron/sl28/sl28.c
@@ -188,7 +188,7 @@
 
 	fdt_fixup_icid(blob);
 
-	if (CONFIG_IS_ENABLED(SL28_SPL_LOADS_OPTEE_BL32)) {
+	if (IS_ENABLED(CONFIG_SL28_SPL_LOADS_OPTEE_BL32)) {
 		node = fdt_node_offset_by_compatible(blob, -1, "linaro,optee-tz");
 		if (node)
 			fdt_set_node_status(blob, node, FDT_STATUS_OKAY);