rust: cpufreq: use core::ffi::CStr method names

Prepare for `core::ffi::CStr` taking the place of `kernel::str::CStr` by
avoid methods that only exist on the latter.

Link: #1075
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Signed-off-by: Tamir Duberstein <tamird@gmail.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: tamird

rust/kernel/cpufreq.rs

@@ -1016,7 +1016,7 @@ impl<T: Driver> Registration<T> {
     };
 
     const fn copy_name(name: &'static CStr) -> [c_char; CPUFREQ_NAME_LEN] {
-        let src = name.as_bytes_with_nul();
+        let src = name.to_bytes_with_nul();
         let mut dst = [0; CPUFREQ_NAME_LEN];
 
         build_assert!(src.len() <= CPUFREQ_NAME_LEN);