rust: io: move module entry to its correct location

The module entry of `io` falsely ended up in the "use" block instead of
the "mod" block, hence move it to its correct location.

Signed-off-by: Danilo Krummrich <dakr@kernel.org>
Link: https://lore.kernel.org/r/20250103164655.96590-3-dakr@kernel.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>