PCI: keystone: Propagate request_irq() failure

Previously, if request_irq() failed, ks_add_pcie_port() always returned
zero (success).  Return the request_irq() failure result instead.

[bhelgaas: changelog]
Signed-off-by: Wei Yongjun <weiyj.lk@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-By: Murali Karicheri <m-karicheri2@ti.com>