xen/pci: Use 'acpi_gsi_to_irq' value unconditionally.

In the past we would only use the function's value if the
returned value was not equal to 'acpi_sci_override_gsi'. Meaning
that the INT_SRV_OVR values for global and source irq were different.
But it is OK to use the function's value even when the global
and source irq are the same.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>