cpufreq: Convert `/// SAFETY` lines to `# Safety` sections

Replace `/// SAFETY` comments in doc comments with proper `# Safety`
sections, as per rustdoc conventions.

Also mark the C FFI callbacks as `unsafe` to correctly reflect their
safety requirements.

Reported-by: Miguel Ojeda <ojeda@kernel.org>
Closes: https://github.com/Rust-for-Linux/linux/issues/1169


Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>