rust: uaccess: take advantage of the prelude and `Result`'s defaults

The `kernel` prelude brings `Result` and the error codes; and the prelude
itself is already available in the examples automatically.

In addition, `Result` already defaults to `T = ()`.

Thus simplify.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20250429151445.438977-1-ojeda@kernel.org


Signed-off-by: Miguel Ojeda <ojeda@kernel.org>