rust: opp: use c_* types via kernel prelude

Update OPP FFI callback signatures to use `c_int` from the `kernel::prelude`,
instead of accessing it via `kernel::ffi::c_int`.

Although these types are defined in a crate named `ffi`, they are re-exported
via the `kernel::prelude` and should be used from there. This aligns with the
Rust-for-Linux coding guidelines and ensures ABI correctness when interfacing
with C code.

Suggested-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Abhinav Ananthu <abhinav.ogl@gmail.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>