PCI/VPD: Add pci_vpd_alloc()

Several users of the VPD API use a fixed-size buffer and read the VPD into
it for further usage. This requires special handling for the case that the
buffer isn't big enough to hold the full VPD data.  Also the buffer is
often allocated on the stack, which isn't too nice.

Add pci_vpd_alloc() to dynamically allocate buffer of the correct size and
read VPD into it.

Link: https://lore.kernel.org/r/955ff598-0021-8446-f856-0c2c077635d7@gmail.com


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