rust: io: fix broken intra-doc links to `platform::Device`

`platform` is not accessible from here.

Thus fix the intra-doc links by qualifying the paths a bit more.

Fixes: 1d0d4b28 ("rust: io: mem: add a generic iomem abstraction")
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Link: https://lore.kernel.org/r/20250722085500.1360401-2-ojeda@kernel.org


Signed-off-by: Danilo Krummrich <dakr@kernel.org>