scsi: fcoe: catch invalid values for the 'enabled' attribute

The 'enabled' sysfs attribute only accepts the values '0' and '1',
so we should error out any other values.

Signed-off-by: Hannes Reinecke <hare@suse.com>
Acked-by: Johannes Thumshirn <jth@kernel.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>