PCI: xilinx: Fix return value in case of error

In xilinx_pcie_init_irq_domain(), the pattern used to check and return
error is:

   if (!var) {
      dev_err(...);
      return PTR_ERR(var);
   }

So the returned value in case of error is always 0, which means 'success'.
Change it to return -ENODEV instead.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Sören Brinkmann <soren.brinkmann@xilinx.com>