pwm: th1520: fix `CLIPPY=1` warning

The Rust kernel code should be kept `CLIPPY=1`-clean [1].

Clippy reports:

    error: this pattern reimplements `Option::unwrap_or`
      --> drivers/pwm/pwm_th1520.rs:64:5
       |
    64 | /     (match ns.checked_mul(rate_hz) {
    65 | |         Some(product) => product,
    66 | |         None => u64::MAX,
    67 | |     }) / NSEC_PER_SEC_U64
       | |______^ help: replace with: `ns.checked_mul(rate_hz).unwrap_or(u64::MAX)`
       |
       = help: for further information visit https://rust-lang.github.io/rust-clippy/rust-1.92.0/index.html#manual_unwrap_or
       = note: `-D clippy::manual-unwrap-or` implied by `-D warnings`
       = help: to override `-D warnings` add `#[allow(clippy::manual_unwrap_or)]`

Applying the suggestion then triggers:

    error: manual saturating arithmetic
      --> drivers/pwm/pwm_th1520.rs:64:5
       |
    64 |     ns.checked_mul(rate_hz).unwrap_or(u64::MAX) / NSEC_PER_SEC_U64
       |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using `saturating_mul`: `ns.saturating_mul(rate_hz)`
       |
       = help: for further information visit https://rust-lang.github.io/rust-clippy/rust-1.92.0/index.html#manual_saturating_arithmetic
       = note: `-D clippy::manual-saturating-arithmetic` implied by `-D warnings`
       = help: to override `-D warnings` add `#[allow(clippy::manual_saturating_arithmetic)]`

Thus fix it by using saturating arithmetic, which simplifies the code
as well.

Link: https://rust-for-linux.com/contributing#submit-checklist-addendum

 [1]
Fixes: e03724aa ("pwm: Add Rust driver for T-HEAD TH1520 SoC")
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Reviewed-by: Danilo Krummrich <dakr@kernel.org>
Reviewed-by: Michal Wilczynski <m.wilczynski@samsung.com>
Link: https://patch.msgid.link/20260121183719.71659-1-ojeda@kernel.org


Signed-off-by: Uwe Kleine-König <ukleinek@kernel.org>