[PATCH] pwm: th1520: fix `CLIPPY=1` warning

[Miguel Ojeda <ojeda@kernel.org>]

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 arithmatic, which simplifies the code
as well.

Link: https://rust-for-linux.com/contributing#submit-checklist-addendum [1]
Fixes: e03724aac758 ("pwm: Add Rust driver for T-HEAD TH1520 SoC")
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
---
It would be nice to clean this up, so that Mark may start enforcing it
in linux-next -- thanks!

Completely untested, so please beware. Also, I am not sure if I am
missing something here, since saturation arithmetic is already used
a few lines below already.

Cc: Mark Brown <broonie@kernel.org>

 drivers/pwm/pwm_th1520.rs | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/drivers/pwm/pwm_th1520.rs b/drivers/pwm/pwm_th1520.rs
index 21b4bdaf0607..fb95f994f963 100644
--- a/drivers/pwm/pwm_th1520.rs
+++ b/drivers/pwm/pwm_th1520.rs
@@ -61,10 +61,7 @@ const fn th1520_pwm_fp(n: u32) -> usize {
 fn ns_to_cycles(ns: u64, rate_hz: u64) -> u64 {
     const NSEC_PER_SEC_U64: u64 = time::NSEC_PER_SEC as u64;

-    (match ns.checked_mul(rate_hz) {
-        Some(product) => product,
-        None => u64::MAX,
-    }) / NSEC_PER_SEC_U64
+    ns.saturating_mul(rate_hz) / NSEC_PER_SEC_U64
 }

 fn cycles_to_ns(cycles: u64, rate_hz: u64) -> u64 {

base-commit: e3b32dcb9f23e3c3927ef3eec6a5842a988fb574
--
2.52.0