fix(aml): Fix clippy::unnecessary_cast

[mkroening]

$ cargo clippy
warning: casting to the same type is unnecessary (`usize` -> `usize`)
   --> src/aml/resource.rs:767:21
    |
767 |     let pin_count = ((source_name_offset - pin_table_offset) / 2) as usize;
    |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `((source_name_offset - pin_table_offset) / 2)`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/main/index.html#unnecessary_cast
    = note: `#[warn(clippy::unnecessary_cast)]` on by default

warning: `acpi` (lib) generated 1 warning (run `cargo clippy --fix --lib -p acpi -- ` to apply 1 suggestion)

While looking for missed warnings in #338, I noticed a new one introduced in c526710. I think this is a good example in favor of #347. :)

[martin-hughes]

Easy tidy up, thanks 😊