pm80xx: do not examine registers for iButton feature if ATTO adapter

ATTO adapters do not support this feature.  If the firmware fails to be
ready, it should not check the examined registers in order to examine
the state of the feature in order to prevent undefined behavior.

Signed-off-by: Benjamin Rood <brood@attotech.com>
Reviewed-by: Jack Wang <jinpu.wang@profitbricks.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>