clk: tegra: periph: Fix error handling and resolve unsigned compare warning

./drivers/clk/tegra/clk-periph.c:59:5-9: WARNING:
Unsigned expression compared with zero: rate < 0

The unsigned long 'rate' variable caused:
- Incorrect handling of negative errors
- Compile warning: "Unsigned expression compared with zero"

Fix by changing to long type and adding req->rate cast.

Signed-off-by: Pei Xiao <[e-mail]>
Link: https://lore.kernel.org/r/7[e-mail]
Acked-by: Thierry Reding <[e-mail]>
Signed-off-by: Stephen Boyd <[e-mail]>

drivers/clk/tegra/clk-periph.c

···
 	struct tegra_clk_periph *periph = to_clk_periph(hw);
 	const struct clk_ops *div_ops = periph->div_ops;
 	struct clk_hw *div_hw = &periph->divider.hw;
-	unsigned long rate;
+	long rate;
 
 	__clk_hw_set_clk(div_hw, hw);
 
···
 	if (rate < 0)
 		return rate;
 
-	req->rate = rate;
+	req->rate = (unsigned long)rate;
 	return 0;
 }