PCI: xilinx: Clear correct MSI set bit

Kernel provides virtual IRQ number at teardown.  Get hwirq number from
virtual IRQ and clear correct MSI set bit.

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>