PCI: pciehp: Acknowledge spurious "cmd completed" event

In case of a spurious "cmd completed", pcie_write_cmd() does not clear it,
but yet expects more "cmd completed" events to be generated.  This does not
happen because the previous (spurious) event has not been acknowledged.
Fix that.

Signed-off-by: Rajat Jain <rajatxjain@gmail.com>
Signed-off-by: Rajat Jain <rajatjain@juniper.net>
Signed-off-by: Guenter Roeck <groeck@juniper.net>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>