scsi: fix comment in struct Scsi_Host definition

Commit 1abf635d (scsi: use 64-bit value for 'max_luns') changed the order
of Scsi_Host members. Update the comment to reflect this.

Signed-off-by: Sebastian Herbszt <herbszt@gmx.de>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>