[SCSI] saved and restore result for timed out commands

Save and restore the scmd->result, so that timed out commands do not
return the result of the TEST UNIT READY or the start/stop commands. Code
is already in place to save and restore the result for the request sense
case.

The previous version of this patch erroneously removed the "if" check,
instead add a comment as to why the "if" is needed.

Signed-off-by: Patrick Mansfield <patmans@us.ibm.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>