PCI: xilinx: Fix xilinx_pcie_assign_msi() return value test

We should be testing "hwirq" instead of "irq".  "irq" is unsigned so it's
never less than zero.  Also it's uninitialized.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Srikanth Thokala <sthokal@xilinx.com>