rust: devres: provide an accessor for the device

Provide an accessor for the Device a Devres instance has been created
with.

For instance, this is useful when registrations want to provide a
&Device<Bound> for a scope that is protected by Devres.

Suggested-by: Benno Lossin <lossin@kernel.org>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Link: https://lore.kernel.org/r/20250713182737.64448-1-dakr@kernel.org


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