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: NRay Jui <rjui@broadcom.com>
Signed-off-by: NBjorn Helgaas <bhelgaas@google.com>
Reviewed-by: NAnup Patel <anup.patel@broadcom.com>
Reviewed-by: NScott Branden <sbranden@broadcom.com>