rust: pci: display symbolic PCI class names

rust: pci: display symbolic PCI class names

The Display implementation for Class was forwarding directly to Debug
printing, resulting in raw hex values instead of PCI Class strings.

Improve things by doing a stringify!() call for each PCI Class item.
This now prints symbolic names such as "DISPLAY_VGA", instead of
"Class(0x030000)". It still falls back to Debug formatting for unknown
class values.

Suggested-by: Danilo Krummrich <dakr@kernel.org>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>