mlx4_core: Stash PCI ID driver_data in mlx4_priv structure

That way we can check flags later on, when we've finished with the
pci_device_id structure.  Also convert the "is VF" flag to an enum:
"Never do in the preprocessor what can be done in C."

Signed-off-by: Roland Dreier <roland@purestorage.com>