usb_update: 1-byte response to extra commands

The existing commands do not really need longer response code.
Future commands can always reply with more data as needed.

BRANCH=none
BUG=b:35587171
TEST=Update hammer using usb_updater2

Change-Id: I37cb8ac444d58216a7d98c9090043b06d95c74e1
Reviewed-on: https://chromium-review.googlesource.com/489887
Commit-Ready: Nicolas Boichat <drinkcat@chromium.org>
Tested-by: Nicolas Boichat <drinkcat@chromium.org>
Reviewed-by: Randall Spangler <rspangler@chromium.org>