rust: io: avoid mentioning private fields in `IoMem`

rust: io: avoid mentioning private fields in `IoMem`

Removed reference to internal variables in the comment of `IoMem`
This avoids using private variable names in public documentation.

Suggested-by: Miguel Ojeda <ojeda@kernel.org>
Link: https://github.com/Rust-for-Linux/linux/issues/1167
Signed-off-by: Sai Vishnu M <saivishnu725@gmail.com>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Link: https://lore.kernel.org/r/20250602164923.48893-2-saivishnu725@gmail.com
[ Reworded title and adjusted tags. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>