rust: pci: move iomap_region() to impl Device<Bound>

Require the Bound device context to be able to call iomap_region() and
iomap_region_sized(). Creating I/O mapping requires the device to be
bound.

Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Link: https://lore.kernel.org/r/20250413173758.12068-8-dakr@kernel.org


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