sfc: 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 avoids having to allocate a buffer on the stack, and we don't have to
make any assumptions on VPD size and location of information in VPD.

Link: https://lore.kernel.org/r/e58f1e40-c043-0266-9a0f-e5a7f3f6883c@gmail.com


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