ACPI, APEI, Fix error return value in apei_map_generic_address()

From commit 6915564dc5a8 ("ACPI: OSL: Change the type of
acpi_os_map_generic_address() return value"),
acpi_os_map_generic_address() will return logical address or NULL
for error, but for ACPI_ADR_SPACE_SYSTEM_IO case, it should be also
return 0 as it's a normal case, but now it will return -ENXIO.

So check it out for such case to avoid einj module initialization
fail.

Fixes: 6915564dc5a8 ("ACPI: OSL: Change the type of acpi_os_map_generic_address() return value")
Cc: <[e-mail]>
Reviewed-by: James Morse <[e-mail]>
Tested-by: Tony Luck <[e-mail]>
Signed-off-by: Aili Yao <[e-mail]>
Signed-off-by: Rafael J. Wysocki <[e-mail]>

drivers/acpi/apei/apei-base.c

···
 	if (rc)
 		return rc;
 
+	/* IO space doesn't need mapping */
+	if (reg->space_id == ACPI_ADR_SPACE_SYSTEM_IO)
+		return 0;
+
 	if (!acpi_os_map_generic_address(reg))
 		return -ENXIO;