PCI: iproc: Allow multiple devices except on PAXC

Commit 943ebae7 ("PCI: iproc: Add PAXC interface support") only allowed
device 0, which is a regression on BCMA-based platforms.

All systems support only one device, a Root Port at 00:00.0, on the root
bus.  PAXC-based systems support only the Root Port (00:00.0) and a single
device (with multiple functions) below it, e.g., 01:00.0, 01:00.1, etc.
Non-PAXC systems support arbitrary devices below the Root Port.

[bhelgaas: changelog, fold in removal of MAX_NUM_PAXC_PF check]
Fixes: 943ebae7

 ("PCI: iproc: Add PAXC interface support")
Reported-by: Rafal Milecki <zajec5@gmail.com>
Signed-off-by: Ray Jui <rjui@broadcom.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>