cmd: scsi: Fix null pointer dereference in 'scsi reset'

During 'scsi reset', scsi_bus_reset() is called with udevice pointed
to NULL, which causes exception. As a temporary fix, disable the call
for DM SCSI for now.

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