[pci] Allow pci_vpd_init() return status to be ignored
Most xxx_init() functions are void functions with no failure cases.
Allow pci_vpd_init() to be used in the same way. (Subsequent calls to
pci_vpd_read() etc. will fail if pci_vpd_init() fails.)
Signed-off-by: Michael Brown <mcb30@ipxe.org>