rust: kernel: remove `fmt!`, fix clippy::uninlined-format-args

rust: kernel: remove `fmt!`, fix clippy::uninlined-format-args

Rather than export a macro that delegates to `core::format_args`, simply
re-export `core::format_args` as `fmt` from the prelude. This exposes
clippy warnings which were previously obscured by this macro, such as:

    warning: variables can be used directly in the `format!` string
      --> ../drivers/cpufreq/rcpufreq_dt.rs:21:43
       |
    21 |     let prop_name = CString::try_from_fmt(fmt!("{}-supply", name)).ok()?;
       |                                           ^^^^^^^^^^^^^^^^^^^^^^^
       |
       = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#uninlined_format_args
       = note: `-W clippy::uninlined-format-args` implied by `-W clippy::all`
       = help: to override `-W clippy::all` add `#[allow(clippy::uninlined_format_args)]`
    help: change this to
       |
    21 -     let prop_name = CString::try_from_fmt(fmt!("{}-supply", name)).ok()?;
    21 +     let prop_name = CString::try_from_fmt(fmt!("{name}-supply")).ok()?;
       |

Thus fix them in the same commit. This could possibly be fixed in two
stages, but the diff is small enough (outside of kernel/str.rs) that I
hope it can be taken in a single commit.

Signed-off-by: Tamir Duberstein <tamird@gmail.com>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Link: https://lore.kernel.org/r/20250704-core-cstr-prepare-v1-1-a91524037783@gmail.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>