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: Tuan Phan <tphan@apm.com>
Signed-off-by: Loc Ho <lho@apm.com>
Signed-off-by: Mark Salter <msalter@redhat.com>
Acked-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>