rust: pci: re-export ConfigSpace

Re-export ConfigSpace, such that users can refer to the type as
kernel::pci::ConfigSpace, rather than kernel::pci::io::ConfigSpace.

Fixes: 4dc0bacb1d3c ("rust: pci: add config space read/write support")
Reported-by: Gary Guo <gary@garyguo.net>
Closes: https://lore.kernel.org/rust-for-linux/DG2D5ONS18FE.TC7K3O8V8SU1@garyguo.net/
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Zijing Zhang <zijing.zhang@ry.rs>
Link: https://patch.msgid.link/995384df9224283fab185b5e06f519506fff1873.1769877524.git.zijing.zhang@ry.rs
[ Slightly reworded commit message. - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>