[SCSI] stex: Fix for potential invalid response

The interrupt routine is good for normal cases. However, if the firmware
is abnormal and returns an invalid response, the driver may reuse a
ccb structure that has already been handled. This may cause problem.
Fix this by setting the req member to NULL. Next time we know the
response is invalid and handle accordingly if req is NULL.

Signed-off-by: Ed Lin <ed.lin@promise.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>