scsi: core: Reduce number of scsi_test_unit_ready() retries

Make scsi_test_unit_ready() send at most as many TURs as specified in
the 'retries' argument instead of retries * (retries + 1) / 2.

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>