buffet: Fix vendor command definition.

Later in the game vendor command were requested to prefix
all the parameters with an underscore.

BUG=None
TEST=manual

Change-Id: I30909e4a7c42feb66365790ee346bb749d3e2bfc
Reviewed-on: https://chromium-review.googlesource.com/223432
Commit-Queue: Anton Muhin <antonm@chromium.org>
Tested-by: Anton Muhin <antonm@chromium.org>
Reviewed-by: Christopher Wiley <wiley@chromium.org>