clk: fix error check for devm_clk_get_optional()

If skip all return error number, it may skip some real error cases,
so only skip the error when the clock is not provided in DTS

Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/clk/clk-uclass.c b/drivers/clk/clk-uclass.c
index 9aa8537..2778b50 100644
--- a/drivers/clk/clk-uclass.c
+++ b/drivers/clk/clk-uclass.c
@@ -678,7 +678,7 @@
 {
 	struct clk *clk = devm_clk_get(dev, id);
 
-	if (IS_ERR(clk))
+	if (PTR_ERR(clk) == -ENODATA)
 		return NULL;
 
 	return clk;