dm: scsi: Document and rename the scsi_scan() parameter

The 'mode' parameter is actually a flag to determine whether to display
a list of devices found during the scan. Rename it to reflect this, add a
function comment and adjust callers to use a boolean.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>