clk: tegra: Don't allow zero clock rate for PLLs

Zero clock rate doesn't make sense for PLLs and tegra-clk driver enters
into infinite loop on trying to calculate PLL parameters for zero rate.
Make code to error out if requested rate is zero.

Originally this trouble was found by Robert Yang while he was trying to
bring up upstream kernel on Samsung Galaxy Tab, which happened due to a
bug in Tegra DRM driver that erroneously sets PLL rate to zero. This
issues came over again recently during of kernel bring up on ASUS TF700T.
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>

drivers/clk/tegra/clk-pll.c

@@ -558,6 +558,9 @@ static int _calc_rate(struct clk_hw *hw, struct tegra_clk_pll_freq_table *cfg,
 	u32 p_div = 0;
 	int ret;
 
+	if (!rate)
+		return -EINVAL;
+
 	switch (parent_rate) {
 	case 12000000:
 	case 26000000: