clk: rockchip: check grf when waiting pll lock

rockchip_clk_get_grf pass on return value from
syscon_regmap_lookup_by_phandle, so we check grf to
make sure whether to do the following things or not.

Signed-off-by: Shawn Lin <[e-mail]>
Signed-off-by: Heiko Stuebner <[e-mail]>

drivers/clk/rockchip/clk-pll.c

···
 	unsigned int val;
 	int delay = 24000000, ret;
 
+	if (IS_ERR(grf)) {
+		pr_err("%s: grf regmap not available\n", __func__);
+		return PTR_ERR(grf);
+	}
+
 	while (delay > 0) {
 		ret = regmap_read(grf, pll->lock_offset, &val);
 		if (ret) {