PCI: xilinx: Don't enable config completion interrupts

The Xilinx AXI bridge for PCI Express device provides interrupts indicating
the completion of config space accesses. We have previously
enabled/unmasked them but do nothing with them besides acknowledge them.

Leave the interrupts masked in order to avoid servicing a large number of
pointless interrupts during boot.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Bharat Kumar Gogada <bharatku@xilinx.com>
Cc: Michal Simek <michal.simek@xilinx.com>
Cc: Ravikiran Gummaluri <rgummal@xilinx.com>