PCI: iproc: Improve link detection logic

Improve the link detection logic by explicitly querying the link status
register to ensure link is active.

Also force class to PCI_CLASS_BRIDGE_PCI (0x0604) through the host
configuration space register.

Signed-off-by: Ray Jui <rjui@broadcom.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Anup Patel <anup.patel@broadcom.com>
Reviewed-by: Scott Branden <sbranden@broadcom.com>