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

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 <xiaopei01@kylinos.cn>
Link: https://lore.kernel.org/r/79c7f01e29876c612e90d6d0157fb1572ca8b3fb.1752046270.git.xiaopei01@kylinos.cn
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>