rust: pci: preserve device context in AsRef

Since device::Device has a generic over its context, preserve this
device context in AsRef.

For instance, when calling pci::Device<Core> the new AsRef implementation
returns device::Device<Core>.

Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Link: https://lore.kernel.org/r/20250413173758.12068-6-dakr@kernel.org


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