scsi_dh_alua: move optimize_stpg evaluation

When the optimize_stpg module option is set we should just set it
once during port_group allocation. Doing so allows us to override
it later with device specific settings.

Reviewed-by: Bart Van Assche <bart.vanassche@sandisk.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>