Correct SPL use of SL28CPLD

This converts 1 usage of this option to the non-SPL form, since there is
no SPL_SL28CPLD 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 4ad6b32..2579236 100644
--- a/board/kontron/sl28/sl28.c
+++ b/board/kontron/sl28/sl28.c
@@ -109,7 +109,7 @@
 int checkboard(void)
 {
 	printf("EL:    %d\n", current_el());
-	if (CONFIG_IS_ENABLED(SL28CPLD))
+	if (IS_ENABLED(CONFIG_SL28CPLD))
 		print_cpld_version();
 
 	return 0;