rust: net::phy represent DeviceId as transparent wrapper over mdio_device_id

rust: net::phy represent DeviceId as transparent wrapper over mdio_device_id

Refactor the DeviceId struct to be a #[repr(transparent)] wrapper
around the C struct bindings::mdio_device_id.

This refactoring is a preparation for enabling the PHY abstractions to
use the RawDeviceId trait.

Acked-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Trevor Gross <tmgross@umich.edu>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
Link: https://lore.kernel.org/r/20250711040947.1252162-3-fujita.tomonori@gmail.com
Signed-off-by: Danilo Krummrich <dakr@kernel.org>