scsi: scsi_debug: skip long delays when ndelay small

A test program's runtime became impractically long since any non zero
ndelay (e.g. 1 nanosec) caused Start Stop Unit to delay over 8 magnitudes
greater than other commands. This patch skips long delays (on Start Stop
Unit and Synchronize Cache) if ndelay is less than or equal to 10
microsecs.

Signed-off-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>