ACPI / irq: Fix return code of acpi_gsi_to_irq()

The function acpi_gsi_to_irq() must return 0 on success as the caller
ghes_probe expects an 0 for success. This change also matches x86
implementation.

This patch was submitted around 4.5 timeframe but wasn't pushed because
it didn't fix a real problem. Now that RAS/GHES patches are in kernel,
this fixes an error seen on a Mustang (arm64) platform:

    GHES: Failed to map GSI to IRQ for generic hardware error source: 2
    GHES: probe of GHES.2 failed with error 81
Signed-off-by: NTuan Phan <tphan@apm.com>
Signed-off-by: NLoc Ho <lho@apm.com>
Signed-off-by: NMark Salter <msalter@redhat.com>
Acked-by: NLorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: NRafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/acpi/irq.c

@@ -24,7 +24,7 @@ static struct fwnode_handle *acpi_gsi_domain_id;
  *
  * irq location updated with irq value [>0 on success, 0 on failure]
  *
- * Returns: linux IRQ number on success (>0)
+ * Returns: 0 on success
  *          -EINVAL on failure
  */
 int acpi_gsi_to_irq(u32 gsi, unsigned int *irq)
@@ -37,7 +37,7 @@ int acpi_gsi_to_irq(u32 gsi, unsigned int *irq)
 	 * *irq == 0 means no mapping, that should
 	 * be reported as a failure
 	 */
-	return (*irq > 0) ? *irq : -EINVAL;
+	return (*irq > 0) ? 0 : -EINVAL;
 }
 EXPORT_SYMBOL_GPL(acpi_gsi_to_irq);