clk: remove redundant negative index check in of_clk_get_parent_name()

This if-block can be dropped because the of_parse_phandle_with_args()
in the following line returns -EINVAL for negative index.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>

drivers/clk/clk.c

@@ -3062,9 +3062,6 @@ const char *of_clk_get_parent_name(struct device_node *np, int index)
 	int count;
 	struct clk *clk;
 
-	if (index < 0)
-		return NULL;
-
 	rc = of_parse_phandle_with_args(np, "clocks", "#clock-cells", index,
 					&clkspec);
 	if (rc)