PCI: iproc: Skip check for legacy IRQ on PAXC buses

PAXC and PAXCv2 buses do not support legacy IRQs so there is no reason to
even try and map them.  Without a change like this, one cannot create VFs
on Nitro ports since legacy interrupts are checked as part of the PCI
device creation process.  Testing on PAXC hardware showed that VFs are
properly created with only the change to not set pcie->map_irq, but just to
be safe the change in iproc_pcie_setup() will ensure that pdev_fixup_irq()
will not panic.

Signed-off-by: Andy Gospodarek <gospo@broadcom.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Ray Jui <ray.jui@broadcom.com>