rust: pwm: Fix broken intra-doc link

rust: pwm: Fix broken intra-doc link

`rustdoc` reports a broken intra-doc link:

    error: unresolved link to `Devres::register`
       --> rust/kernel/pwm.rs:722:11
        |
    722 | /// via [`Devres::register`]. This ties the lifetime of the PWM chip registration
        |           ^^^^^^^^^^^^^^^^ no item named `Devres` in scope
        |
        = note: `-D rustdoc::broken-intra-doc-links` implied by `-D warnings`
        = help: to override `-D warnings` add `#[allow(rustdoc::broken_intra_doc_links)]`

Thus fix it.

Fixes: d8046cd50879 ("rust: pwm: Add complete abstraction layer")
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Link: https://patch.msgid.link/20251029181940.780629-1-ojeda@kernel.org
Signed-off-by: Uwe Kleine-König <ukleinek@kernel.org>