tg3: Set 10_100_ONLY flag for additional 10/100 Mbps devices

- Also refactor the conditional to use the existing tg3_pci_tbl array.
- Set flags in the driver_data field of the pci_device_id structure to
identify these devices.
- Add PCI_DEVICE_SUB() to pci.h to declare PCI 4-part IDs to match these
devices.

Signed-off-by: Nithin Nayak Sujir <nsujir@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>