PCI: xilinx: Clear interrupt register for invalid interrupt

The interrupt decode register is not being cleared if an invalid interrupt
arises.  Clear the decode register in this case.

Signed-off-by: Bharat Kumar Gogada <bharatku@xilinx.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Michal Simek <michal.simek@xilinx.com>