pci: intel: Fix error when enumerating multi-function PCIe device

Hardware return completion status non-zero when read from non exist
function in multi-function PCIe device. Return error will cause PCIe
enumeration fail.

Change it to return 0 and return value 0xffffffff when error.

Signed-off-by: Ley Foon Tan <ley.foon.tan@intel.com>