rust: net::phy always define device_table in module_phy_driver macro

rust: net::phy always define device_table in module_phy_driver macro

device_table in module_phy_driver macro is defined only when the
driver is built as a module. So a PHY driver imports phy::DeviceId
module in the following way then hits `unused import` warning when
it's compiled as built-in:

 use kernel::net::phy::DeviceId;

 kernel::module_phy_driver! {
     drivers: [PhyQT2025],
     device_table: [
        DeviceId::new_with_driver::<PhyQT2025>(),
     ],

Put device_table in a const. It's not included in the kernel image if
unused (when the driver is compiled as built-in), and the compiler
doesn't complain.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://patch.msgid.link/20240930134038.1309-1-fujita.tomonori@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>