PCI: xgene: Fix platform_get_irq() error handling

When platform_get_irq() fails we should propagate the real error value
instead of always returning -EINVAL.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Duc Dang <dhdang@apm.com>