rust: error: allow `useless_conversion` for 32-bit builds

For the new Rust support for 32-bit arm [1], Clippy warns:

error: useless conversion to the same type: `i32`
--> rust/kernel/error.rs:139:36
|
139 | unsafe { bindings::ERR_PTR(self.0.into()) as *mut _ }
| ^^^^^^^^^^^^^ help: consider removing `.into()`: `self.0`
|
= help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#useless_conversion
= note: `-D clippy::useless-conversion` implied by `-D warnings`
= help: to override `-D warnings` add `#[allow(clippy::useless_conversion)]`

The `self.0.into()` converts an `c_int` into `ERR_PTR`'s parameter
which is a `c_long`. Thus, both types are `i32` in 32-bit. Therefore,
allow it for those architectures.

Link: https://lore.kernel.org/rust-for-linux/2dbd1491-149d-443c-9802-75786a6a3b73@gmail.com/ [1]
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Christian Schrefl <chrisi.schrefl@gmail.com>
Link: https://lore.kernel.org/r/20240730155702.1110144-1-ojeda@kernel.org
[ Fixed typo in tag. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>