PCI: designware: Relax device number checking to allow SR-IOV

Previously we only allowed device 0 to be directly attached to the root
port.  But SR-IOV devices may use non-zero device numbers for VFs.

Remove the restriction that only device 0 may be attached to a root port.

[bhelgaas: changelog]
Signed-off-by: Po Liu <po.liu@nxp.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Jingoo Han <jingoohan1@gmail.com>