ACPI / APEI: adjust a local variable type in ghes_ioremap_pfn_irq()

Match up with what 7edda088 ("acpi: apei: handle SEA notification
type for ARMv8") did for ghes_ioremap_pfn_nmi().

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>