rust: cpufreq: use c_ types from kernel prelude

Update cpufreq FFI callback signatures to use `c_int` from the `kernel::prelude`,
rather than accessing it explicitly through `kernel::ffi::c_int`.

Although these types are defined in the `ffi` crate, they are re-exported
via `kernel::prelude`. This aligns with the Rust-for-Linux coding
guidelines and ensures proper C ABI compatibility across platforms.

Signed-off-by: Abhinav Ananthu <abhinav.ogl@gmail.com>
Suggested-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
[ Viresh: Fixed rustfmtcheck errors ]
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>