This reverts commit a1cbfd55.

Test isn't useless.  scsi_req_enqueue() may finish the request (will
actually happen for requests which don't trigger any I/O such as
INQUIRY), then call usb_msd_command_complete() which in turn will
set s->req to NULL after unref'ing it.
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>