PM / devfreq: tegra30: Check whether clk_round_rate() returns zero rate

EMC clock is always-on and can't be zero. Check whether clk_round_rate()
returns zero rate and error out if it does. It can return zero if clock
tree isn't initialized properly.

Acked-by: Chanwoo Choi <[e-mail]>
Signed-off-by: Dmitry Osipenko <[e-mail]>
Signed-off-by: Viresh Kumar <[e-mail]>

drivers/devfreq/tegra30-devfreq.c

···
 		return err;
 
 	rate = clk_round_rate(tegra->emc_clock, ULONG_MAX);
-	if (rate < 0) {
+	if (rate <= 0) {
 		dev_err(&pdev->dev, "Failed to round clock rate: %ld\n", rate);
-		return rate;
+		return rate ?: -EINVAL;
 	}
 
 	tegra->max_freq = rate / KHZ;