Since the wwn_to_u64() function does not change its input, make its
argument const.

Cc: James Smart <james.smart@broadcom.com>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Johannes Thumshirn <jthumshirn@suse.de>
Cc: Steffen Maier <maier@linux.vnet.ibm.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>