rust: cpufreq: clean new `clippy::map_or_identity` lint for Rust 1.98.0

rust: cpufreq: clean new `clippy::map_or_identity` lint for Rust 1.98.0

Starting with Rust 1.98.0 (expected 2026-08-20), Clippy is likely
introducing a new lint `clippy::map_or_identity` [1][2], which currently
triggers in a single case:

    warning: expression can be simplified using `Result::unwrap_or()`
        --> rust/kernel/cpufreq.rs:1326:60
         |
    1326 |         PolicyCpu::from_cpu(cpu_id).map_or(0, |mut policy| T::get(&mut policy).map_or(0, |f| f))
         |                                                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
         |
         = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#map_or_identity
         = note: `-W clippy::map-or-identity` implied by `-W clippy::all`
         = help: to override `-W clippy::all` add `#[allow(clippy::map_or_identity)]`
    help: consider using `unwrap_or`
         |
    1326 -         PolicyCpu::from_cpu(cpu_id).map_or(0, |mut policy| T::get(&mut policy).map_or(0, |f| f))
    1326 +         PolicyCpu::from_cpu(cpu_id).map_or(0, |mut policy| T::get(&mut policy).unwrap_or(0))
         |

The suggestion is valid, thus clean it up.

Cc: stable@vger.kernel.org # Needed in 6.18.y and later.
Link: https://github.com/rust-lang/rust-clippy/issues/15801
Link: https://github.com/rust-lang/rust-clippy/pull/16052
Reviewed-by: Zhongqiu Han <zhongqiu.han@oss.qualcomm.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Link: https://patch.msgid.link/20260530095809.213611-1-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>