tg3: Read VPD with pci_vpd_alloc()

Use pci_vpd_alloc() to dynamically allocate a properly sized buffer and
read the full VPD data into it.

This simplifies the code, and we no longer have to make assumptions about
VPD size.

Link: https://lore.kernel.org/r/bd3cd19c-b74f-9704-5786-476bf35ab5de@gmail.com


Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>