scripts: rust: replace length checks with match

scripts: rust: replace length checks with match

Use a match expression with slice patterns instead of length checks and
indexing. The result is more idiomatic, which is a better example for
future Rust code authors.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Tamir Duberstein <tamird@gmail.com>
Link: https://lore.kernel.org/r/20250529-idiomatic-match-slice-v2-1-4925ca2f1550@gmail.com
[ Reworded title. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>