PCI: iproc: Reject unconfigured physical functions from PAXC

PAXC is an emulated PCIe root complex internally in various Broadcom
based SoCs. PAXC internally connects to the embedded network processor
within these SoCs, with the embedeed network processor exposed as an
endpoint device.

The number of physical functions from the embedded network processor
that can be accessed depends on the firmware configuration.

Unfortunately, due to an ASIC bug, unconfigured physical functions cannot
be properly hidden from the root complex during enumerattion. As a
result, config write access to these unconfigured physical functions
during enumeration will cause a bus lock up on the embedded network
processor.

Fortunately, these unconfigured physical functions contain a very
specific, staled PCIe device ID 0x168e. By making use of this device ID,
one is able to terminate the enumeration early in the vendor/device ID
config read.

Signed-off-by: Ray Jui <ray.jui@broadcom.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Scott Branden <scott.branden@broadcom.com>
Reviewed-by: Oza Pawandeep <poza@codeaurora.org>