scsi: scsi_debug: IMMED related delay adjustments

A patch titled: "[PATCH v2] scsi_debug: implement IMMED bit" introduced
long delays to the Start stop unit (SSU) and Synchronize cache (SC)
commands when the IMMED bit is clear.  This patch makes those delays
more realistic. It causes SSU to only delay when the start stop state is
changed; SC only delays when there's been a write since the previous
SC. It also reduced the SC delay from 1 second to 50 milliseconds.

Signed-off-by: Douglas Gilbert <dgilbert@interlog.com>
Tested-by: Ming Lei <ming.lei@redhat.com>
Reported-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>