Correct SPL use of SPLASH_SOURCE

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

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/common/splash.c b/common/splash.c
index 2e466a8..245ff68 100644
--- a/common/splash.c
+++ b/common/splash.c
@@ -89,7 +89,7 @@
 
 __weak int splash_screen_prepare(void)
 {
-	if (CONFIG_IS_ENABLED(SPLASH_SOURCE))
+	if (IS_ENABLED(CONFIG_SPLASH_SOURCE))
 		return splash_source_load(default_splash_locations,
 					  ARRAY_SIZE(default_splash_locations));